After quite a few batches I would say 21 is the ideal, my batches at 21 tend to be the largest and the most healthy. I have had some held as long as 30, which I force at that point. The fry always tend to be underdeveloped and not as many when it gets much past 23 days or so.
So I would suggest, that if all is going well they will be released in great numbers about that 21 days. After that, you have something not going well. As noted, the papa tends to get more aggressive, faster after that point.
At 21 days, I find that removal of the father in 12-18 hours adequate. After that, he seems to be willing to go after they fry, even at the cost of the ones still in his mouth...as time extends, the damage increases.