When they eat and drink on their own. I clean the cage, then add the new ones and the existing ones. They all seem to co-exist happily. I do this so the parents have alone and stress free time
I do it when I feel like it honestly.
Experience tells me when they need to be moved. I keep mine with the parents for a long time, so that they learn social skills early on and can be colonized later on in life easily. My main goal is temperament, so that's why I do this.
I take everyone out at the same time, and throw them all in one tank together. If there are any that start trouble (there are usually 2-3 out of 20+) I take them out and feed them off. The rest are further checked and handled to see who is best suited for pets.