Unless you need to be separated from your baby early on or circumstances require you to start pumping immediately (such as having a preemie in the NICU), it is recommended to wait about 4-6 weeks before you begin pumping.
During these first few weeks your body is acclimating to the needs of your baby. When your milk supply comes in originally it is usually a lot more than you actually need. With time your body adjusts to the rhythm and just-right quantity of milk to make for your baby.
Every time your baby comes to the breast your body is signaled to make more milk. The entire breastmilk production process is based on demand. The more you effectively nurse your baby, the more milk your body will make. Emptying your breasts of milk tells your body that it’s time to make more milk. Conversely, when your breasts are full your body slows down milk production.
Adding in additional pumping sessions during the first few weeks can affect the flow that your body is working towards to make the exact amount of milk needed for your baby. For some people this could even lead to an oversupply of milk which has its own complications.
Once your body has had these few weeks to regulate and fine tune the breast milk production process, it is a good time to begin to enter in pumping.