How often you dust with D3 depends not only on how much natural outdoors in the sun time he gets, but also on the brand of supplement* you are using and the type of UVB bulbs you use, and the density of material between the bulb and the cham (screen blocks some UVB).
*Rep-Cal has 400,000 IU/kg of D3 whereas Miner-All (I) has 4,400 IU/kg of D3 and I think 150IU D3 in Nutribol (which also has Vit A so be wary of using this very often)
That's a considerable difference in D3 strength.
I dust lightly with phos.-free calcium with d3 supplement about twice a month. My chameleons never go outside. I've historically used ReptiGlo 5.0 bulbs with very little between the cham and the bulb (no screen mesh).