i assumed it was 1, as bob grew up very fast, then after hitting 1 didnt grow much (hes 1 and 3 months now) but then while i was on holiday he had a sudden growth spurt and when i got back he'd grown into his fat (he was pretty chubby when i left)! lol. he had becove a very long pig, and he wasnt exactly small to begin with, hes always been a big pig.
i found that munchie was growing very slowly compared to bob, but he suddenly grew last week, and is now exactly the same size as bob, and hes 3 months younger, so i think its different with each piggy. the size they can grow to differs too i think, especially if you get a breed like a rex in my experience, just because of how munchie is built, and the fact that he is still growing, compared to bob