Don't know how long guppy has been pregnant, when to seperate?

I noticed my female guppy beginning to look pregnant last week, but I don't know when the actual fertilization began, so I don't know exactly how far along in pregnancy she is. I don't want to move her from the tank too soon, so she doesn't have to be seperated and stressed out longer then she needs to be. Any tips on when I should put her into the breeding box? Thanks so much for your time and energy, I really appreciate it.