When these mothers used their hands as well as their pump to express milk, they pumped an average of 48 percent more milk than the pump alone could remove. According to another study, this milk also contained twice as much fat as when mothers used only the pump. - read later
Naptime Tales: Pumping Schedule for nursing and exclusively pumping. Personally I would pump until 3-5 minutes after the milk stops flowing, not for a set amount of time. If you're still getting milk after 20 minutes, KEEP PUMPING!!! Also, it shouldn't hurt to pump. Discomfort maybe, pain, no. Something is wrong if there is pain.