I believe that the typical bloom period for a pawpaw tree usually occurs over 3 to 4 weeks. If that is the case for your cultivars, I would pollinate after a good amount of blooms have opened and polinate the previously unopened blooms when the previously pollinate blooms pass their peak bloom period. My guess is about every 5 -7 days. An alternative method is to mark pollinated blooms and don't re-pollinate those. Use a small marker dot on the branch to mark them.

I used to pollinate my pawpaw when the trees were young and a person could reach the flowers. Used to get a decent crop.
Now that the flowers are way too high I couldn't be bothered...and the crop is bountiful. Nature does it best.