It varies a lot.
Controversial topics that can easily be posted in with a few words and not much thought get a lot of traffic. They'll last from five hours to a couple of days.
Controversial topics that require a little more brainpower will last maybe three hours to a day, depending on how controversial they are. The Escapist is an opinionated lot, after all.
Any particularily difficult topic that does not contain much controversy will be completely ignored, for the most part. It'll probably sink away within the first five minutes.
A non-controversial simple topic is a lot more difficult to determine. It really depends on how interested people are in the subject.
Sometimes, it doesn't even have to be particularily interesting. Make a thread that asks about bellybuttons or something and it will probably go on for maybe seven pages.
However, pages do not determine how long it lasts timewise. It will get posted in a lot, but how long it stays around is entirely up to chance.
I like to think of these as the "wild card" topics. They can, potentially, last longer than any of the other types of topic. It has a lot to do with luck and how The Escapist is feeling that week.
TL;DR I OVERANALYZE EVERYTHING