Depends massively, sometimes I can get a whole past paper done in one go, sometimes I only get one or 2 questions in and I get distracted.
I don't ever set myself time limits, or arbitrary limits on what I must do each day, that only gets me stressed. What I do is each day set myself some work and either do a solid paper (or 2) and mark it, or I'll set a stack of past questions in front of me (half my exams have very few past papers so my teachers have made collections of past questions that are stll relevent).
If I set myself questions rather than papers I'll spend quite a few hours "revising" but half of that time is spent trawling the internet.