Put simply, in their natural environment, most chameleons eat >50 variety of insects. All of these insects have differend diets themselves and thus have differing ratios of calcium to phospherous. Example-Snails have tons of calcium vs phospherous. Crickets have less calcium vs phospherous. Chameleons need there blood serum to contain a 2:1 ratio to maintain good health. This is not even taking into account all of the other essential nutrients and vitamins they are provided.
Frankly, in captivity, you are not feeding the variety they are getting in the wild. If you are feeding primarily crickets, this can be a staple diet, but modifications need to be made.
Your question is a reasonable one. My suggestion is that you give as much information on the cage, food, lighting that you provide as well as the type of chameleon (age and sex included) and some other keepers can give you a better idea of how to provide a multivitamin and calcium (with +/- D3) and how often.