Hi YoungKrypt
Kitties only need food to survive, the milk is for when they're breeding to raise their happiness and in turn their love and speed up the process (but the same can be achieved by interacting with them, like cuddling, or owner-petting for 30 minutes every day, or no interaction and waiting a few more days).
Kitties stop breeding at 121 days old. That said, if you want it as a pet only you can turn breeding off and they will eat half as a breeding kitty. You can turn back breeding on at any moment, but you'll have to (a) wait for a week until it begins t ogain love again or (b) give them a pet recovery vitamin for instant effect.
1 bowl of food (number 1 food) lasts 1 breeding kitty 4 weeks and one pet kitty 8 weeks.
Kitties come with a toy to play with their owner, just go in their menu to interact --> play. Bear in mind that not all toys that you see in the market are interactive. Kitties love the attention!!!