I'm making a game that's a parody of a well known game. I want to take a a few assets from the original game to evoke the player's memory of the original game. What are good guidelines to use for how much is appropriate? So far I think I want a 4 second audio sample from the main theme and maybe a still shot of a side character's face. Is that too much? Reasonable? My game is only useful for this game jam so it won't be sold or anything.

First rule of any copyright question is "Ask a lawyer"
I am not a lawyer, let alone your lawyer.
Parody is covered under fair use, money doesn't come into it, the idea that it matters at all in the question of "is this fair use?" is internet lies.

Even if it wasn't an explicit parody:
Audio sampling is almost certainly fine, because remixing audio is pretty permissive, the screenshot might not be?
Also, software license agreements to do with games tend to have a big old "EFF NO, DON'T DO THAT" clause for reproduction using the software

EDIT: I am not a lawyer and as such I may have said something inaccurate RE Audio sampling.
 Turns out there are separate licenses for sampling for remixes of music tracks.

So yeah, on the whole: don't try to skirt copyright.

Thanks. I've posted my game already and the assets I reused should be obviously parody. In any case my game is free, unlisted in the search engine, and it says that it is intended only for the audience of this game jam, so it should be fine and if not it's easy to take down.