It’s difficult to say really. It depends on the age and whether they have had fry before in my experience. My molly was not very big at all when she had her first lot of fry and I was told she had weeks left but I thought she looked square-ish and had a separate tank, so I moved her anyway. She then had the fry within the next two days. She also did not display any signs of labour, such as hiding and not eating. However, she is pregnant again with her second lot of fry and she is very big and has squared off. So look for the stomach going square in shape and that means they should be ready soon. I agree though that a breeder box is very stressful and can cause you to lose the fry. If you want to make sure they live and need to move the mothers out, is there a possibility for getting another tank? This would also be a place to keep the fry with more space than a breeder box