D.W.
Wait until around 7-8 weeks if you can to start pumping as baby is getting your supply to meet his demands at this point.
Your breasts are adjusting to breastfeeding so that's why you don't feel as full as you did in those first days and again, your supply is getting regulated. The good news in that baby takes about the same amount of milk per day from around age 1 month to age 6 months. Around 6 months baby will hit a pretty good growth spurt and will nurse like crazy to up your supply. Of course there are other growth spurts along the way too but 6 months is usually a biggie. Please don't think you don't have enough milk! If you follow baby's lead and put him to breast when he tells you that what he needs you will do just fine!
Please know that what you pump and what baby gets can be 2 very different things. Baby is far more efficient at the breast than the pump.
It is ideal to thaw the breastmilk in the fridge and then gently warm in a cup of warm water prior to feeding baby.
Because breastmilk is rich in antibodies you can use the leftover milk for the next feeding but no longer than that. You can not do that with formula as it has no antibodies whatsoever.
Check out kellymom.com There is a calculator on there somewhere that will help you estimate the amount of pumped milk baby will need while you are away from him.