Well I wouldn't bump anything over a year old, unless the topic is relevant.
Relevancy to the thread and current events play a part, that's for sure. Bumping a four year old thread about the burger you had would probably not be so necessary, but bumping, let's say, a three year old thread about something I can't come up with that is relevant to current events would be plausible. I'd suppose it's up to you to figure out what is too old. I'd like to say anything past four months old from the last post probably shouldn't be bumped up.
It's basically what Sparro said. I've never taken issue with someone bumping a thread that still has discussion value regardless of how old the thread is, especially in a board like Brewster's where there's a vast array of topics. The only time it's really a problem is if the topic has no relevancy at all (i.e. bumping someone's 2 year old shop thread when the person hasn't been online in nearly as long) or when someone goes on a bumping spree and posts in multiple old threads just for the sake of it. As long as the topic is somewhat relevant and the post is both contributory and adds to the discussion, there's normally no problem with bumping.
uhhh it depends on what it is i guess and if there's a big thread for it (like with the lgbt thread that's not super active but bumping it is always ok i guess lmao)
and if the discussion is dead/not relevant anymore it is usually better to make a new thread
but, like 1 month old threads are ok to bump if you have a reason to bump it, but 3 month old threads are a bit :| in my opinion