Set up before a clock, and every minute on the minute perform 5 pull-ups, 10 push-ups, and 15 squats. Can you continue for 30 minutes? 20 minutes? How about 10? Post results to comments. If you fall behind the clock, keep going for 30 minutes and see how many rounds you can complete.
If you've finished the workout before, this time add 1 rep to each exercise, (i.e., 6 pull-ups, 11 push-ups, and 16 squats) every minute and see if you can go the full 30 minutes. We've seen +3. Any +4s?