To my knowledge, it is not possible to pump too often, as long as the pumping is not physically hurting you. It's very common for moms to feel like they have more milk if they allow it to build up. But waiting for milk to build up is a short term gain, long term loss. Short term, you see more milk. Long term, your body gets the message that there isn't as much demand as there used to be, and supply goes down as a result.