Here's how it goes:
Socks: Wear once and then wash. I like to walk around the house in my socks, so they get all kinds of crap on them.
Underpants: Wear once and wash. I'd rather wear none at all than wear the same pair twice, so if I'm out of clean pants, I go commando.
Shirts: I can wear the same shirt for two days in a row, unless it's really sweaty and horrible. I tend not to, though, since I have a lot of shirts and I don't run out very often.
Jumpers/Sweaters: As long as it takes to get a stain on them. Usually between 5-10 minutes.
Jeans/Trousers: Two days.