How long are you supposed to manually pump each breast? When I did it, I would stop when there…

How long are you supposed to manually pump each breast?

When I did it, I would stop when there wasn't very much coming out anymore.. It would take me like 15-20 minutes for each breast.

Since then, I've dried up... I'm trying to figure out why. Could it be that I didn't pump long enough each time?

It also could be that I didn't pump every three hours exactly...?