In one study, researchers found that the maximum milk flow rate occurred at either the first or second letdown 94% of the time. Most of the milk was removed in the first two letdowns so aim to pump until at least 2 letdowns (or more) are observed, then continue until drops of milk no longer flow for 2-5 minutes. One way to stimulate more letdowns is through breast massage/compressions.
Doing breast compressions while pumping can help stimulate additional let-downs, and help to thoroughly drain your breasts. While you’re pumping, use one hand to massage your breast from the armpits towards the nipple (or as close as you can get without dislodging the pump flange). Alternate with a few firm squeezes of your breasts, feeling for areas of fullness.
Finish off the pump session with a few minutes of hand expression to remove those last few creamy drops of milk.