Accutane pregnancy test timeframe

My daughter has been on accutane since March.. her last appointment (with pregnancy test) was July 15th.. her next appointment is August 14th. Our dilemma is she has a schedule conflict and I might need to change the appointment. Problem with that is,her dermatologist is always backed up several weeks.. how many days can she go between her last pregnancy test (July 15th) and the next one before she would not qualify for the medication and would have to start the whole 2 negative test process like we did in the beginning?