Hi dear, if you keep your breast milk in the fridge, you should use it within 48 hours. In Singapore's weather, I will use it by the next day. For freezer it can last 2-3 weeks I think, but I try to use them in a week.
How much to pump will depend on your supply. A general rule is to pump around 15min - 30min, or until your breasts are empty (they feel so soft like they've been deflated).
I use a warmer to warm up the milk when needed. It takes me about 10 - 15minutes to warm up the milk.
Since you have you've just given birth, I assume you are exclusively latching on your baby. Thus you should pump if you've skipped a feeding.
If you need to supplement with FM, feed her breast milk first (through your breasts or bottle), followed by FM. dont't mix the 2.
Finally, try to latch on your daughter at every feeding as it will be essential to establish your milk supply. You may, however, wish to pump 3-4 times a day after a feed to help boost the supply. But this is optional.