you mean after 30min when you stop the milk is still flowing? or do you mean after you stop your breasts still feel full?
usually if milk is still flowing (after letdown reflex) i will let if flow finish before i stop which would take roughly another 2-3min after the 15min session per breast.
actually why dont't you latch your baby on directly? it can stimulate the supply better. baby's suckling is always better than the pump.