it is different between chams but when they are younger generally once a month. Also it is common for the cham to eat the skin after they shed, so if you are not home when it happens, there may not be any evidence that he shed. =)
my newest cham finally shed with me, and it took him about 6 hours to get all his skin off.
If you do notice him shedding try and make sure the humidity is up and everything as it does help a lot.