I don't think there is a true guideline... there are just some things to consider...
In my experience the fry grow much quicker and much fatter under the parents care than under my care. Even when I put them in a dedicated fry tank with "ideal" filtration, "ideal" water parameters, 6~8 daily feedings of live hatched BBS, with daily tank cleanings... I guess I just don't have the momma gene in me...
So leaving them with the parents has it's advantages...
But I don't eat the fry, so pulling them out has it's advantages as well...
So I think getting to know your pair and learning their patterns to leave the fry with the parents for as long as possible, then right before the parents start eating their own fry, remove them...
The earliest I've removed them were the day before the eggs hatched... the latest I've waited was when the offspring started pairing off and spawning...
Forgive the 2 am humor... But like you imagined, there is no "guideline", it's a matter of getting to know your pair and learning what works best with/for them...