Hi Lockshockbarrel,
It depends on how often you feed them and to some degree their genetics. They grow, and move up in prey size, rather quickly if you feed them often- say every 5 days or so. Also, if you have a snake with a genetic predisposition for growth you may have to move the prey size up sooner than for one without.
I have two snakes that will be one year old this fall. They are currently on 1-2 smallish fuzzies every seven days. Until three weeks ago they were on only 1 fuzzy every week and clearly not always being filled up by the smaller meal. I bet if I had power fed them every 4-5 days, as some people do, they would be quite a bit larger and possibly already eating small mice.
I hope this helps.