Were doing the switch to whole milk i need advice?

my daughter will be 1 on monday. I bought a half gallon of whole milk while at the store today. I gave her about 4ozs in a bottle and she drank most of it. I still have about 2-3 cans of formula left over. Do babies get confused if i were to still give her formula like to keep in the diaper bag for going out trips? and milk while at home? we are out and about often so dont have a way to store the milk. Also my other question is how long does a 2 gallon jug of milk usually last? or how many do you end up buying a week. Does 2 gallons last the whole week? Any signs i should be watching for now that she has drank the whole milk