
Ideally it shouldn't start until the dog has been successfully health-checked, so any time after 12 months (hip-score time) would be acceptable for the first mating. It is sensible to wait 3 months or so till his second mating, and then not used more than once a month or so until he is 18 months to 2 years.
Of course it also depends on whether bitch owners want to use him!