It depends. I can go long times without looking at the thread, in which case I just skim the last few posts.
When I'm reading the thread regularly, I try to read everything since the last visit.
I've got to be more careful, though...A few times recently, I posted entries, and on reflection recalled that I had posted the exact same damned thing long ago (a year or more). I left the stupid things in, but am trying to avoid further "senior moments".