For the purposes of this poll 'chivalrous' means they at least try to live up to the stereotypical knightly virtues (with varying degrees of success): courage, honor, courtesy, justice, and a readiness to help the weak.

Explain your choice if you have the time.

I like Knights, and frankly, playing villains is easy.  So I have at least one a campaign, if there's no actually knightly orders in it, more if there are full on castles and stuff.

I love knights, but I tend to tilt the camera a bit because a perfectly chivalrous knight in shining armor is somewhat limited in their potential narrative application.

So, I've got a few I roll out as NPCs from time to time.

Sir Parzavel, an older knight with long and tangled red hair and beard shot with white, battered and rusty harness, a notched sword with a makeshift grip wrapped with waxed twine, and a ill tempered nag of a misshapen horse.  He's the greatest paladin in the, world of course, fearless, noble, and utterly unaffected by greed or pride.  He's a great way to slip a ringer into a party, it's funny how rarely they expect anything from him.  One of my favorite stories about him is the time the party was fleeing from a greater demon, and he looked at the young knight in the party with a gleam in his eye as he turned to hold the way, "well kid, are you coming or not?"  

The Baron of Bastion is a loyal retainer of the local evil king.  He is himself a kindly lord who eats only fruit, vegetables, course bread, and gruel at feasts.  In his defense he claims to have lost three stone since he went off rich food.  In reality he refuses to eat better than the least of his vassals.  He is a gracious host, a just and kindly lord, and utterly loyal to the oaths he swore to the evil king.  While he is getting on in years and not a particularly mighty warrior, his men are fiercely loyal and very well trained.

Lord Brenner of Brenner manor is a former paladin of the gods of light.  He now dabbles in the dark arts and seeks alliances with their enemies.  He is fond of his dogs and has as little to do with his vassals as possible.  His chamberlain handles his domain competently and fairly while trying desperately to turn aside the eyes of the church and neighbouring nobles.  Brenner is not so much an evil man as a man who has rejected the gods and seeks their downfall on ethical bounds but as his search for the power to withstand them draws him deeper and deeper into the darkness problems are sure to arise.

Black Dannor is a robber knight who waylays passersby for ransom and loot.  He sees himself as a valiant rebel fighting against the evil empire.  And often speaks courteously to those he robs and imprisons without cause.  For all that he his no coward and no fool.  Nor will he fight for a dishonorable lord or be forced to surrender to such.  He is a top notch fighter and his men are well trained dirty fighters.  His depredations are well within his rights by some interpretations of the local laws.

It's pretty damn rare. Sure, I'll have the occasional NPC who tries to live up to virtues like that, but they're as likely, if not more so, to be a shopkeeper, tradesman, noble, or something else entirely as they are to be some kind of professional combatant.

I voted 'often' in that there seem to be some in every campaign.
My Karameikos 'Palaces & Princesses' game is full of them, from the Grand Duke down. A couple are the Duke's Herald Sir Jonathan 'Old Ironbones' Tendoros, and Sir Gabrionus, the Baron Vorloi - father of the PC Knight Lady Alexandra Vorloi.
My Curse of the Crimson Throne game had the Hellknight Sir Jereth Rogare.
My Wilderlands game has the Paladin Malenn of Mitra.
My Loudwater FR game has a bunch of local knights, and the knights of Elturgard, notably Princess Shaedra Kyatt.

My recently ended 5e + Harn game featured two knights stolen straight out of the Trollbridge scenario. I played them as honorable and strong with chivalry, honor and a touch of pomp and they served as an effective front-line for the melee-shy PCs.

One was quasi-promised to one of the female PCs who was the daughter of a baron. Her father could not tame her so allowed her to "go on an adventure" to the east marches with his considerable entourage.

Well, it ended up being an actual adventure. :-) She gradually warmed up to him but her predilections and membership in the Lia Kavair (thieve's guild) was pulling her in a more... independent direction.

He never besmirched her honor and treated her as the princess he hoped she would someday be.

In the future I think I will avoid scenarios that have NPCs in charge. I could NOT get rid of these damn knights, the PCs kept healing 'em and saving 'em and the knights returned in kind. There literally was no threat to these adventurers as the knights handled combats decisively especially with the support of two thieves, a mage, and a priest in the wings supporting them.

Knights. Can't live with 'em - can't kill 'em.

Seems to come up fairly regularly with players and NPCs and as a DM I use them for some campaigns.

Quite a bit (I chose 'often') because of my campaigns, almost all have involved social intrigue/roleplaying in a courtly milieu to some extent (and the current game I'm running is about 50% politics in a fairly standard Medieval Fantasy Realm).  Generally I don't like the simplistic portrayal of nobility or other powerful people as simply corrupt/nepotistic/tyrannical and lording it over an oppressed peasantry.  I try to portray knights and lords as people with a diverse range of personalities, some venal and others earnestly trying to live up to the ideals of their society.  Generally they're a mixed bag---for example in my longest-running game the main society was mostly an adaption of ancient Rome so there wasn't exactly a cultural equivalent to the 'Knight in Shining Armor,' but there was a group of famous heroes and while most of them were kinda hams used to the adulation of the masses they genuinely tried to protect people and do good.  In the current game, which is a solo campaign, my friend is playing a Knight Banneret which is basically like junior-tier nobility and he has a rivalry with this other guy who technically has the same rank but who's father is part of the peerage and looks down his nose at those bootstrapping themselves into the court from the lower gentry.  Yet that guy is a war hero who deserves his acclaim and tries to uphold the ideals of chivalry, he's just kind of an asshole in some ways. (Also he's technically a Rogue/Fighter just because I wanted to give him a distinctive fighting style).

We don't play fantasy inspired by the medieval era, so pretty much never.

Quite often.  A large part of my campaign is set in the elven empire, where "errantry" is a common thing among the young -- more or less the ideal of Arthurian knight-errantry, where you head out under a pseudonym with only what you can carry on your back to Do Good and Right Wrongs.  The concept runs throughout the culture, if sometimes more in the breach than in the observance.

I don't do a lot of campaigns where knights are common. When I run games with Knights, they tend to be on the gritty side.

I am running a wuxia campaign right now, and the heroes in that genre are sometimes translated as Knight Errants because there is a code of honor at work (though it is a bit of stretch to compare the two---they are not tied to lords like knights, though they may be tied to a sect, teacher, etc). In these campaigns, the codes are important to most of the NPC martial heroes they meet but rarely do the players meet one who lives up to every ideal. They are more likely to encounter someone who lives up to one or two of the ideals but seriously falls short or radically reinterprets one of the others. But that is also because bad guys are often more prominent in the campaign.

Many times, but it's certainly not the majority, so I put "rarely". I and the people I've played with almost always invent our own cultures, and the European chivalric ideal appears sometimes, but not always, and even when it's there, there tend to be far more knights who don't live that way.

I'd say they were very frequent if you meant to include warriors who follow their honor code, as we have far more of those who aren't chivalric per se. Samurai who respect bushido, Wood Wardens who protect the forests, Knights of Tniotkea who follow the Blue Code, etc.

In Warhammer, absolutely yes. In a land of corruption, chivalry and honor is rebellion against the status quo.

In my OD&D, rarely if ever.

Kinda the same here. On BX knights were few and far between. But in AD&D they popped up fairly regularly.

Depends where we are on the "break conventions"/"nothing wrong with stereotypical" pendulum, and also whether we're in typical medieval Europe.