I thought it was 6 months but for Meecho that is when she had some freakish growth spurt. she is now 17 months old and I haven't noticed any changes for the last 2-3 months. So maybe a year or around there.
Mews2much is right. Every cat is different, especially when you're comparing two cats of different sized breeds.
Cats gain most of their size within the first 8 months of age. But large cats. such as Maine Coons, will continue to put size on exponentially until they're about 1.5 years and then they'll start filling out. Sphynx, in general, aren't a big breed. So, most of their size will be obtained within the first 8 months as well, at which point they'll do more filling out. Of course, then there's the aspect of expecting a boy to grow to a bigger size than a girl.