Some very experienced keepers I know cycle their times with what is their chams natural environment i.e. Madagascar times for panthers, etc. Maybe that's going a little overboard unless your talking first generation WC chams.
In my experience, I've seen that somehow the chams know the light cycles even without total window exposure to the outside. Generally, I would try to emulate what's going on outside. Like here in Texas, in the middle of summer it's light out from about 7 to 8:30 or 9. In the winter, it's 7:30 to 6:30 or 7. If your chams have window access, try to set it closer to what they are seeing.
As a general rule, I go 13 hours in the summer and 10 or so in the winter. It gives them some emulation of seasons.