Don't waste it!
Once your baby has sucked on a bottle you can no longer save the leftover milk. So if you're pumping 5 oz but your baby only eats 4 oz, put 4 oz in the bottle; if he's still hungry give him the remaining ounce, but if he's full you can freeze the leftover. Breastmilk last 3-4 months in a standard freezer. You can use it in a bottle later or mix it into some rice cereal.