A female will drop her fry rather regularly until she is done. She might pause for a few minutes but that is about all. If you do not see a new fry in a period of around 10 minutes, she is done. At tat point you can do whatever it is you intend to do to preserve fry. In my case that means removing the female from the drop tank. If you use a breeder trap, it is time to return the female to the main tank.