So what do you do for the days where there are more than 12 hours of sun? I thought you were supposed to have 12 hours on and 12 hours off?
This is where I get confused on the whole light thing.
The 12 hours of darkness is a good guideline and can be used year-round.
It comes close to what African chameleons would have in the wild.
Equally as important, I suspect, is to ensure that lighting is consistent, rather than doing things like lights on at 6am today but on at 9am tomorrow--lights off at 4pm today then off at 7pm tomorrow, etc..
That would never happen in nature.
Chams that spend all their time outdoors do get less sleep in the summer than in the winter--but the change is gradual--by a minute or two each day.
Giving your indoor chams 12 hours of darkness will ensure that they get enough rest to stay healthy and is closest to their natural schedule.
In Madascar, where many chams are found, the shortest day of the year is 11 hours of light, the longest day is just over 13 hours of light---so a 12 on 12 off schedule is pretty close to this.
In the US and in Europe, our day lengths vary much more.
As the days grow longer, you may find that your chams get up earlier, despite your lighting coming on later, so some folks adjust the schedule to mimic the local day length more.
Some people even have staggered lighting schedules in which some lights come on and turn off before others to simulate dawn and twilight lighting.
Consistent, long enough rest periods will help ensure your cham gets enough rest to support good health.