what cloth do you use? P: I think I ought to get my little guy one, as it gets chilly at night AND he's in the kitchen where my brother will keep him up all night when he gets home from college
I usually follow the same rule as my frogs, light on when the sun comes up, light off when it starts to get close to time for evening to fall so that they have time for their eyes to ajust instead of sudden darkness P: since you use a blanket, maybe leave the room lights off if there's a window nearby, or turn his light off ( if he has one ) and then turn the room light off in slow succession to give the feel of the sun going down, light getting less XD then leave the lights off till it gets fully dark, and just cover it up with the sheet in the dark. then you can turn the lights back on
if you can't use that method then just ease it over the tank perhaps so it gets darker and darker as evening sets in, then eventually goes all dark? ^-^; if you don't care about it being natural succession then I'd say just when it gets dark outside. same way they probably sleep in the wild naturally

general rule: 12 hours of light, 12 hours of night ^-^