Sets stay together until I need parts from them. Sometimes if its a set I really like, and I only need a few parts from it, it will stay together and I will just modify it a bit to work without whatever part I took. If I need a lot of parts from it, or just don't think its that cool of a set, I'll just take it apart completely. Same thing with MOCs. Some MOCs stay together for only a few days, and I have others that have been assembled for over a year (Though many of the older ones I've modified over time, fitting in new building techniques and parts and such to improve them, so I only have 1 or 2 that are over a year old and still unchanged.
Big sets are a bit different though. I always keep my largest set assembled, until I get a bigger one. Currently its the Tower Bridge, before that it was the Death Star playset. With big MOCs, though, I will probably take those apart pretty quicky after photographing them and such, since they are so part consuming, and take a lot of space. My SHIP I'm working on now will probably only survive a month or so past finishing.