Generally 12 hours of light a day is recommended, and if light from a window shines onto the cham's enclosure, it's probably best to keep your light schedule close to the hours of daylight outside so that the cham doesn't get confused. I like to adjust the length of time that I keep the lights on gradually as the seasons progress.
This is from a previous post of mine:
I did some research on the web, and compiled the table below. It shows the amount of daylight hours (from sunrise to sunset) in the capitals of Madagascar (where most species are from) and Yemen (where the Veiled species orginates) on the 15th of each month of the year.
Month Yemen Madagascar
------ ------ ------------
Jan.... 11h19... 13h09
Feb.... 11h39... 12h42
Mar.... 12h02... 12h13
Apr.... 12h29... 11h40
May... 12h51... 11h13
Jun.... 13h02... 11h00
Jul...... 12h58... 11h06
Aug.... 12h39... 11h38
Sep.... 12h13... 11h59
Oct.... 11h48... 12h30
Nov.... 11h25... 13h00
Dec.... 11h03... 13h16
This might serve as a guide as to how much light to provide to your chameleons throughout the year. As you can see, 12hrs is quite a good approximation. And perhaps this can be adjusted to give an hour's extra light in summer, and an hour less in winter.
I have timers on my lights that switch them on for 11 hours a day in winter, and gradually over the months I increase that to 13 hours a day by peak summer (it correlates roughly to the actual hours of daylight in South Africa).
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One of the best pieces of advice I got concerning lighting was from Zerah Morris and Will Hayward, who told me to set up the lights on separate timers, so that you can provide cues of darkness to the cham.
I have my lights set up on individual timers so that the UV bulbs come on 30mins before the heat bulb, and then turn off 30 mins after the heat bulb. When the heat bulb turns off, the chams know that it is time to sleep and they all move to their favourite sleeping perch before the UV bulb goes off.
This would simulate the natural heating up and cooling down periods at dawn and dusk. When the temperature cools down, the chams take it as a sign that night is approaching.