If the snake can eat/swallow a fuzzy, it will eat a fuzzy, otherwise it won't. It's that simple. If it won't eat the fuzzy, it won't eat the fuzzy. You get a fuzzy and try it, to see where you are at, so to speak.
I have hatchlings that have been eating fuzzies since 14 or 15 inches, often every 3, 4, or 5 days, sometimes less often. That's probably a reasonable minimum size for attempting a fuzzy. Maybe I don't know what the heck I'm talking about, but I do think there is something to the idea that baby animals eat more often, and this GRADUALLY changes (not jumping from eating every 7 days to every 14 days, for example.) Also, the idea of someone feeding a 20 inch snake 1 pinky per week seems absurd to me. Others have posted info about fuzzies being more nutritious than pinkies, so the effect of getting away from pinkies is beneficial in more ways than simply the mass/volume of food.
If you think your snake might be ready for a fuzzy (or even if you don't, and the snake is over 15 inches), try it. At most, you're out $1 or whatever you pay for the fuzzy. Try again in another month or 6 weeks, if your first attempt is unsuccessful (highly unlikely, based on my experiences.) Snakes haven't survived this long from lack of ability to eat without human supervision and intervention. They know what they can and can't handle.
Some wild Cornsnakes surely must eat BETTER in the wild (compared to their poor captive cousins), based on some posts I see. There are lizards and frogs all over my yard. Any wild snakes here would be fit and well-fed, not stunted and waiting 7 days for a single pinky. If food were that scarce, it is doubtful Corns would inhabit such a geographic region. Sure, they CAN go without food for weeks or subsist on meager rations, but I'm not sure that's what nature actually intended. They'd probably look for a different location with more food, after several weeks or months of hunger or meager little meals. Like I said, I have thousands of lizards and frogs in my yard here, and this is just a drop in the bucket. Anybody that thinks snakes can't find food in the woods (like we have in Florida, the Carolinas, etc.) hasn't been in the woods lately.