Well, since all my dogs are rescues who are well into their adult coat, I had to go looking.
It seems to range between 4 months (or so of course) on up to around a year - apparently depending on the dog. What you'll see first is the presence of longer hair - his guard hair - coming in over his juvenile hair, then he'll gradually lose that juvenile hair -or- he may just blow the juvenile coat as his adult undercoat comes in.
I'm not a lot of help since, as I said my dogs were all adults when I got them and looking on the web doesn't seem to give anything resembling a definitive answer.
“Properly trained, a man can be dog’s best friend.”