Multiple let down question

So I've noticed when I'm power pumping that I have 4-5 let downs over the whole hour. I'm convinced if I kept pumping, I'd get more let downs. Everyone says to pump until breast is empty but when is it actually empty? When I get no more let downs? Or when it feels empty? On a regular pump I usually try to achieve 3 let downs and then stop (about 20 minutes) and my breasts generally feel empty. What's the best thing to do?