I think it depends in part on the wood you use, and the type of smoke you're getting. 3-5 hours of thin blue smoke is fine with a medium or mild wood (imho). 3-5 hours of thick white smoke billowing out of every pore is guaranteed to oversmoke.
More pungent woods (like hickory and mesquite) should have less time for a given level of "smokiness". Mild woods (like most fruit woods) take more.
If your smoke goes like mine usually do, you might get 30-60 minutes of heavier white smoke, then very thin (almost impercetable) smoke for a couple hours, with the occasional 15-20 minute burst of white smoke. That seems to work well for me.