How many push ups is 'to failure'?
I do Calisthenics morning and night too, and have for years.
"To get the most out of exercise, you should monitor your intensity throughout your workout. This will insure that you are working within your heart rate training zone, thus getting an effective workout without killing yourself.
THR describes the pulse rate (in beats per minute) that allows you to exercise safely while getting the maximum benefits from your workout. This range is usually between 50% to 85% of your maximum heart rate. You can calculate target heart rate with any number of formulas..." - http://exercise.about.com/cs/cardioworkouts/a/ex_intensity.htm
Most exercise physiologists will tell you that getting your THR up several times a day for 20 minutes or longer is the best way to improve overall fitness.