I think it's just a common courtesy to personally respond to any purposed trade, one way or another. I agree with the thought that if someone can log on, they can take a minute and respond. A "sorry I don't have time. I don't like the trade. Can't find the cards." at least something!
An ignored trade or even a denied trade with no explanation leaves me scratching my head. If you have cards on your want list and on your trade list, I assume you are looking to trade. I've gone to the trouble of seeking you out, pulling the cards and putting together what I feel is a fair proposal, then crickets. I don't get it. It seems very rude.
As to the original question as to how long I let a trade sit, I don't necessarily have a set time. I think there are a lot of factors, but I don't think I'd exceed 2 weeks.