This will vary from species to species. In general most babies (less than a year old) should be fed an appropriately sized prey item every seven days. And then from there move it back to every 10 days or so, then back to every 2 weeks. Most adults are best fed every 2 weeks or so. You should watch the individual animal, if he is getting a bit on the obese side, for that particular species, cut back a bit. Many species of snakes can be "power fed", meaning trying to grow them quickly, without much negative consequence, but I don't think it is a good idea, they never seem as healthy as adults, and tend to be more prone to health problems. It also seems to be beneficial to allow a snake to have a full digestive cycle before feeding again. This also allows to see how long it takes to digest, and make sure they defecate properly. It's not a hard and fast rule, but it is a good idea with most species.
All of this is a general idea, you have to take the particular species of snake into account. Green tree pythons are generally fed a little less often, as their metabolism seem to move a little slower, and they are a bit more prone to digestive problems. Carpet pythons have high metabolisms and you almost can't overfeed them. Emerald tree boas have a very unusual digestive process, as compared to most captive snakes, and only defecate on average every 3-4 weeks, even if fed every 7-10 days, and are prone to a number of digestive problems, one being coined "emerald regurgitation syndrome", ERS, or "emerald vomiting syndrome", EVS. And this is not clearly understood yet on a scientific level.
Add to this that it's not always as simple as "feed the snake", sometimes a particular animal won't eat for various reasons, some won't eat when going into a shed cycle, or at certain times of year, (brumation, breeding times, etc...) or because of stress, etc...
I have numerous animals, and numerous species, and I keep a detailed feeding record, and for some, a detailed defecation record. It is the only way to keep track.
