How long do you breastfeed

How long do you breastfeed your baby? I know we should not count how long and let baby feed but when my baby was 1-3 months he would spend a while nursing. Now it takes maybe 5 min on one side and then I offer the other one, I am not sure if is because he can get milk faster or maybe he doesn't want to wait for the let down. He is 4 months now.