It depends on the individual, I've had a male yellowbelly that bred at 5 months of age, he produced a couple of clutches that year and then the following year he wouldn't lock up at all and didn't breed again until he was 3 years old.
Males do not need to be locked up for hours at a time to be successful, I would think that a few minutes is really all they need. Like I said, I've had a male that I never saw locked, I've got other males that I rarely see locked up but they still get the job done. One thing you can do is check for evidence the next day, breeding couples will often leave what look like little blood trails and musk smears on the paper (another reason I like using newspaper as a substrate)