First get the idea that they will finish in 60 days right out of your head. Chances are it will take longer; breeder flowering times are notoriously inaccurate. Not only that peak potency is subjective; its at the preference of the grower.
That being said after you flip the lights check the nodal areas daily for wispy white pistils to emerge. It will take approx 6-10 days for the transition to occur but on the day you see early flowers forming mark your calendar with a star or something. THIS is day one. Most hybrids or indica doms will finish in 9 weeks or so...that is 63 days. If you count 9 or 10 weeks out from day one that's a good estimate of when you might be harvesting or at least give you an idea of when your harvest window will be open. You still need to check the color of the trichome heads to determine maturity.