When they are big enough not to be eaten. Ideally they would be better in a quarantine tank, but its not always possible. Its not great to keep them in a breeding trap - if its a small one.
It will be a month or 2 before they are big enough to be let out. You can buy net partitions for the tank, which would be a better idea really, that way you dont stunt their development, they get better swimmers and are more likely to hunt out food. My LFS sells the stick in dividers, which are about £8 for a smallish one.