Unless you want to get all scientific and do regular blood test on the lizard, id stick to it.
Depends on how much time outside you can give it. Now your approaching warmer months, more time outside, you might like to cut back, but in winter when you just can't, then stick to the D3. *shrugs*
Folk will argue amounts, but I like to rely on nature where possible. There is no exact answer for you mate.
Unless you live in Florida like I do! My chameleon is basically outside everday if weather is permitting. Yesterday he was outside all day long. I only give the d3 once or twice a month and that is probably not even necessary with all the natural exposure he gets to the sun
I'm starting to bring my chams outside more and more, but I'm going to to stick with my supplement schedule. The chams are good at regulating their UVB needs, so if they don't need it, they will just hide from my UVB bulb.