The general recommendation that seems to work for most women is that for the first 12 weeks to pump around 8 to 12 times a day. The length of each session should be around 15 minutes or 5 minutes past your last let-down, whichever is the longest. In order to get your 8-12 sessions in one day, that means that you will pump each 2 or 3 hours in 24 hours. You can have closer pumping times during the day and longer stretches during the night.
These guidelines will help you increase your milk supply. By pumping so often what you end up doing is sending a signal to your body to produce more milk. Every time you empty your breasts, they start producing again.
The first 12 weeks are important as it is believed that during this time your supply is more easily influenced as your Prolactin levels (hormone primarily associated with lactation) are highest.