I recently started doing these three times a week. I usually do 20 push-ups, crunches, then 20 pushups.
So 40 I guess. Sunday night I'll see if I can do a straight 50 without killing myself.
I had a High school English teacher who happened to be a coach. If you were late he'd make you do push-ups to whatever the minute hand was on. Class began at 10:25. I got in shape that semester.