Depends on the dog. Most drop pretty early, but it could be much longer. I have a male that I'd actually scheduled for neuter...then 2 weeks before, one of his 'little marbles' showed up. He was a week short of a year old. So I postponed the neuter to wait for the second. Gave up, rescheduled and sure enough, a week before, the second 'little marble' showed up. He was 16 months old when the second one dropped.
I've heard that it sometimes takes longer with smaller males...not sure if there is any truth to that. Mine is 3 pounds, soaking wet, so it made seems to fit in his particular case.