rule of thumb, one year.  But that is only a generalization.  Maine Coon's
don't reach full maturity till year three.

While in general, cats are all the same size, they do vary from five to
fifteen pounds (or twenty five if you have someone who over feeds)  As long
as your kitty is active, and has good input and output, Im sure he's just
going to be a big one.

Final size is determined mostly by heredity and nutrition (including
prenatal nutrition).  My largest normal cat is more than twice the
weight of my smallest.  At one point I had three from the same litter,
one was tiny (under six pounds) and the other two were large (about 10
pounds) - the litter were born on 9 Sept. and by the end of January,
the little one was nearly as large as her mother, and the two males
were the same size as their mother.
<http://gearbox.maem.umr.edu/tdavis/cats/020205/corrie.mudpie.junk.2.600x400.jpg>
shows the mother and her look-alike son when the son was six days
short of five months (I don't know which is which).

Most kittens appear to stop growing in about a year or year and a
half, but some, Main Coon cats in particular, may continue to grow for
up to five years.

I have four cats. Smokey is a 2yo half burmese. Phil and Lil (1.5) are heinz
siblings from the same litter. Boof 10 months) is Lils offspring. Lil is
almost dainty. She is a small petite cat barely making it to 7 pounds. Phil
seems to be a well grown male and weighs about 11 pounds. Boof grew like a
weed until he was Lils height and length and appeared to stop at about 8-9
months. In the last few weeks (maybe because spring is here?) he is ever so
slowly growing. He is now a fraction taller and longer than Lil, but doesn't
have the muscle mass of his uncle Phil - yet. Smokey is as long and not
quite as tall as Phil, and also doesn't have the musculature, but she isn't
related.