If you consider their native habitat, Latitude is 13 to 17 degrees. Which means that there is not much variation on sunlight hours. So I just leave mine at 12 on / 12 off, 365 days. Also being in Florida, having the chams by the window doesn't affect their light cycle as those of you who live further north with the 18/6 hr light during the summer solstice.
I have timers for all my reptiles, and they all get a very minimal increase/decrease in summer/winter.
For 6 months of winter they are on at 9:30 and off at 8.
For 6 months of summer, they are on at 8:30 and off at 10:30.
It's also good to have different lights coming on and going off throughout the day, intensifying the light. You could do this for the summer only too.
For example: You usually have one linear 5.0 UVB tube going, and one heat bulb going (60-75w). Those should be on all day, but adding a 7.0 UVB tube and another 40w heat bulb that are on for 3 hours of afternoon can be beneficial for light exposure, (short term) uvb exposure, and heat for thermoregulation.
If you do this, it's best to feed in the morning, so the added heat can help in digestion and uptake of the nutrition.
You don't HAVE to do this, but it might be something to consider during the summer months. It kinda simulates sunrise, high noon, and sunset.