Try a full workout once and you'll figure it out. You'll realize that a weight that seemed easy for about 5 reps at the beginning will be plenty sufficient after 15 or 20 minutes.
And check out "Your 10 Most Common EDT Questions Answered":
7) Is there a certain number of reps I should be getting in each PR Zone?
Sort of. I hate people to get too wrapped up in looking for an "ideal" number of reps, but since I know they do, I'll provide some suggestions. GENERALLY, I like people to hit about 60-70 reps in a PR Zone on the first go-around. This means 60-70 reps on both exercises.