I want to start building a milk stash.

I’ve been exclusively breast feeding my son for two and a half weeks. I want to start building a year of milk so he can be fully off the boob by 4 months(I know it may take longer and I may have to still pump at the 4 month mark to reach that goal). How do I go about building a stash ? How often should I be pumping?

I don’t want to breastfeed past 4 months cuz I just don’t so don’t ask.