How much abdominal exercises per week are needed to maintain tone for a 25 year old female?

Unanswerable: First i doubt that this is question for a medical site. Exercise is so personal and so varied and the outcomes are so different. There is no story to your question. As a physician when discussing exercise i would put abdominal exercises at the bottom of the list for effective exercise. One needs big muscle groups moving and cardiovascular exercise to keep toned and healthy.