Muscle maintainance (pushups/crunches)?

I am 15, and I do 400 push-ups a day and 160 crunches/situps (half in the morning, half at night b/c that's the only time I can do them). I have been doing this for 7 1/2 months now to help my heart and chronich chostocondritis, and I just am to tired to keep on doing that many each day. What is a good number to do to maintain my muscle and such? (i am fine with the situps though)