Everyone is different, there is no single answer to this question.
I disagree. If there is a basic standard to achieve, then it's clear to see if they're "good enough" with a particular gun.
The hard part is agreeing on a basic standard. I think it's worthy pursuit and means much more than someone naively declaring they "feel" confident in their ability, especially when they can't recall the last time they shot the gun they're carrying.
The standard should involve a realistic distance, acceptable accuracy and enough speed. Of these, distance is the easiest to agree upon, but acceptable accuracy ranges from a golf ball to the entire black of a B-27 target.
Speed ranges from meaningless descriptions like "as fast as necessary," or "as fast as I can," to a lethargic time frame that has no bearing whatsoever in a defensive shooting situation. (The 5x5 and 3-3-3 drills come to mind)
I posted my drill and some folks didn't like it, but I don't recall them offering a viable alternative.
I'll even modify it: gun in hand at low ready. At signal, fire at a sheet of typing paper OR paper plate until two rounds hit. Two seconds max, but can shoot as many rounds as needed to get your two hits....but two seconds maximum time, regardless of rounds fired.