How much Breast milk to prepare for a night out?

Lyla
Hi all, 
My birthday is coming up and my MIL has offered to babysit for a couple of hours whilst we have a meal out. She did actually offer a weekend away but I declined. 
Anyway I EBF so if I was to prepare bottles how many Oz on average will a 3 month old need?
I do have milk stored in the freezer and I know I would need to start giving bottles earlier to ensure she would take it. 
So how much per feed should I start with?