You definitely don't need to give him it that often. Use calcium without d3 at every feeding, calcium with d3 once every two weeks and a multivitamin once every two weeks.
Dusting with a phos - free calcium /D3 powder twice a month will ensure that your chameleon gets some D3 without overdosing it and leaving it to produce the rest from its exposure to the UVB. D3 from supplements can build up in the system and lead to health issues.
it depends!
what brand of supplement are you using? (some are far more potent that others - some can be used a few times a week, others only a couple times a month)
what type of chameleon?
Does it go outside?
etc
Chameleons housed outdoors all the time dont usually need added D3 to their diets, though providing a small amount now and then "just in case" is alright too