If you intended to latch your newborn, usually every 2-3hrs or when LO cry for milk/latch. Additionally you can use hand to express or pump whichever you prefer(I only use hand express, never used a breast pump), when LO is sleeping exceeded the latching time or when you feel that your boobs is “full/filled” with breastmilk. The more you latch, the more your bobs will produce breastmilk.
As frequent as you breastfeed, or in between pumps if you are looking to try to gradually increase your milk supply