Probably texas holdem (is most likely, though it's up to the host), and probably a good three or four hours, and it's usual to have a set limit and 'buy' chips at the start (say £20). Either you each have your own colour, and then buy back your chips at the end (so you might have spent £40) from whoever has them, and sell chips that arn't your colour back. or you can keep chips for the next time you play.
i would suggest a Friday, or Saturday night being a better option (but am open to other days).
A friend told me of what (imho) sounded like a good way to play simple, low stakes poker.
They got a lot of chips and divided them equally between all the players at the start of each session. A player pays, say, £10 for their chips (£10 being a number determined by how much you want to play for, rather than the number of chips to hand). If you have a lot of chips, then the game goes on for longer, but the cost is still the same.
You set a time limit on each game (say 1 or 1.5 hrs. Enough so that you can play a few games in an evening, and so that if you go out early in the game you aren't hanging around with nothing to do for ages). The game ends either when the time limit runs out, or when one person has won all the chips. Winner takes the total of everyones' stakes (or, if you hit the time limit and there are two or three players still left in the game), then whoever is left in splits the total stake money in the proportion of the number of chips they are left with in relation to the total number of chips that you started off with.
What I particularly like about this (thinking about how useless I'm likely to turn out to be) is that if you go out you can still look forward to another game eventually.