How often are you supposed to change the nipples? How do you know what size fits best to your baby?

My baby uses the avent bottles and it says online that size 0m+ or 1m+ is ideal for breastfeeding babies of all ages. But 3m+ says that it’s for babies 3 months and up, which my baby is going to be 4 months this Saturday. I’m not sure which one to get since I breastfeed my baby when I’m home and my mother bottle feeds him when I’m at work. Please help