The roo needs to have hit rooster puberty, so.. crowing, mounting, and that's going to be a range for each individual bird. Also first mating attempts can be a bit awkward til the guy gets his balance, and the ladies accept his advances. Our rooster was interested pretty young, but the hens wouldn't go for it until they were mature enough to lay eggs (it was only a couple weeks, but the poor guy was a bit frustrated trying to figure out what to do).
 
I'd say watch him to see if he's successfully mating, and then after a week, check a few eggs for bull's-eye spots. If you're consistently seeing fertile eggs from your hens, you know he's been doing his job. We waited about 6 months after our hens started laying to try incubation, and since we'd been eating the eggs and seeing fertile spots on them, we were pretty sure things were good in that department.