I have an 8 year old with mild cerebral palsy, how long will she need physical therapy... She goes once a week right now and it's really helping.

Continue. She should continue with physical therapy as long as it helps. As she gets older, some physical activities will take the place of the PT, but do not rush to end it if she is still benefiting.
Variable issue. Physical therapy is designed to improve motor function to the potential of the patient & assist in acquiring the best method of mobility. Although the defect of CP is static, the need for some form of therapy may remain for some time as the kid grows. Each case varies in need & you may find that at some point you and the kid can do what you must do to maintain the gains of therapy.
SOME THOTS. Really glad this is helping, and would anticipate at some point in near future there would be a transition to an independent home program. Might confer with the therapist and discuss goals, and anticipated outcomes. Together design a template to address progress and newer approaches. Typically, these days, insurance companies may provide some limitations, so check on this, but continue program.