It varies and it's determined by heredity. Add to that the fact that 'puberty' is a pretty nebulous term when trying to use it in terms of time. You can't say, "My puberty started at this time, and ended here." It's impossible to pinpoint the onset and completion of puberty. However, there is more concrete information about when mature cock size is usually attained. According to a study conducted a few years ago by three universities, over 90% attain full genital growth by the age of 16. Apparently many of the dudes on this board are exceptions. Personally, I reached my full dick size by the time I was 15, so I fall within the norm.