You have a little male, right? If not, disregard what I've written, because it'll be different for a female.
In my opinion this is a gradual change that happens over several months. If at 3 months a chameleon is eating 20 small crickets, by 4 months he's eating maybe 15 medium crickets, and then by 5 he's eating 10 medium-largish crickets, and by 6-7 months he's down to 5-7 large crickets, for example. Obviously it depends on the size of your chameleon, as some 6-7 month olds might still not be big enough for large crickets, but you get the rough idea.
But generally, by 7-8 months you can probably have them on 5-7 adult crickets and then a couple months later switch to feeding every other day. It really depends on your chameleon, his size, and growth. I wouldn't cut off a chameleon that seems to be staying very small, for example.