The insects will still have the same calcium to phosphorous ratio even when the chameleon is full grown (unless you change what you feed/gutload them with), so you will still have to dust them with a phos.-free calcium powder...forever. It cuts down naturally in that you feed them less often. Unless you go really crazy with the dusting, calcium won't build up in the system. Remember that the amount of calcium the chameleon will be able to use hinges on the amount of D3 available.
From what I understand, D3 produced from exposure to UVB shouldn't build up (the chameleon's system should regulate it) as long as the chameleon can move away from the UVB source to allow the chameleon to regulate its exposure to it. It doesn't take a lot of hours of sunlight to provide enough UVB...thus D3.
Here's a site that talks about UVB exposure...
http://www.uvguide.co.uk/whatreptilesneed.htm
Beta carotene sources of vitamin A are safe (won't cause an overdose) too...you have to be careful with the preformed though.
The D3 from supplements and the preformed vitamin A are two that build up in the system and need to be watched the closest.
For an adult veiled or panther chameleon I dust the insects with the phos-free calcium powder at most feedings. I dust the insects with a vitamin powder containing a beta carotene source of vitamin A lightly twice a month and I dust with a phos.-free calcium/D3 powder lightly twice a month. I feed the crickets with an assortment of greens and veggies.
Please be aware that if you use animal prey such as mice/pinkies it will already contain preformed vitamin A and D3. Take into consideration anything you use to feed your insects regarding calcium, phos., D3 and preformed vitamin A when you are trying to balance supplements.