I've had mbuna mouthbrood for as little as 14 days to as many as 35 days.
18-21 days seems to be the norm. Some skillful mothers manage ways to eat small food particles despite having a mouthful of fry. Inexperienced mothers will swallow some or all their fry until they're able to endure the fasting period.
They've adapted to go without food for this duration and really aren't 'starving' (I'd get concerned if they went passed 35 days, that' would probably be starving).