Depends on the 'quality' of the feeding. Out of every hundred or so hatchlings with some species I'll end up with a couple which I have absolutely no trouble feeding unassisted, but without good technique, newbies generally won't be able to feed them. Sometimes they get to around a year old and I still routinely feed them in about 15 seconds with a thawed rodent from forceps, but it's just that I know the snake and how that snake needs to mouse to be presented. Even though it's easy for me and the snake might have had around 50 unassisted feeds, I don't consider it ready to sell to a novice.
At the other extreme, some of them absolutely smash their first feed without hesitation, and with some species you just know if they do that on their first feed it is only a formality to give them any more before sale.
I wish it was as easy as just getting them to eat unassisted 4-6 times because if you're really good at feeding snakes you can get them to that stage very very easily (but many of them still won't be ready).