FTM here. How do we know when we can keep breastmilk in bottle in fridge vs in milk bag in freezer?

It also depends on your supply vs. baby's demand. My supply was always catching up with my baby's feedings so mostly we just stored in the fridge. If you find that you have an oversupply and adding another bottle to the fridge means some of the others will spoil before getting consumed, then just freeze the next batch you pump. You also freeze to increase your "stash" for when you're planning to go back to work or go on a trip.