You want to pump as frequently as you would nurse your baby. You can just toss it since the baby would not be able to use it. When you pump after there is no more milk coming out just pump for another minute so it tells your body to keep producing more milk, otherwise it might start to decrease the amount you produce.
Just as "dawniespreckels" answered, you do want to pump a much as possible but since you can't pump constantly remember that milk is also produced from the babies suckling or stimulation of the nipples. So when your not pumping, use your hand to stimulate the nipples. Hope this helped. Veronica B.