As an artist, I try not to let people wait more than a week or two, depending on my situation at home. I expect about the same from other artists but have waited upwards of six months at which time I got annoyed at them and cancelled the trade because they were doing art for other people and they were getting theirs, they just didn't actually want to work on my art because they were 'lazy' as they said.
Item trades, I prefer quick sales and quick buys, so I tend not to wait more than a few hours unless otherwise agreed upon by both parties.
I don't think I've ever had a trade open for more than a few days, because I've always dealt with people who are active enough to get back to the trade within that amount of time. I can't see myself letting a trade just SIT there, though...I'm so impatient. XD I'd probably cancel it after maybe two weeks or so, that seems fair unless there are circumstances preventing the other user from filling their end for some reason.
For me it depends on what I'm waiting for and if the user has been on withing a decent amount of time...if I'm waiting on art and there's been a week or more without the user logging in and posting updates at all, I usually withdraw my order and cancel the trade. For items, I wait about a week.
Depends. If its a friend, I'll usually hold a trade longer, only because I know they'll update me on whats going on.
If its anyone else, it depends. I can usually hold up to about..2 weeks or so. But, I also keep an eye on the person that asked for an item to be held. Like, if I see them running around the exchange, spending gold, that they didn't have for my item, then I'll cancel the trade right away.