Commited physical culturists aren't any different than anyone else who has a lot to do and often times have a limited amount to exercise. However, as one who is devoted to doing some kind of organized exercise as often as possible, I don't look at having only 15-20 minutes to train as a death sentence to training at all. I take that alloted time that I have and figure out a way to make it work most effectively for me. Otherwise, I'd probably never train when work gets busy. It goes back to my father-in-law's favorite saying: "don't give me problems, give me solutions!"
One of the keys to making a short period of time a sweaty, torturous nightmare for your muscles is intensity. You've got to keep things going and going hard to get the most out of the time that you have. You can do this by minimizing breaks. If I know that I'm short on time for a workout, I'll do my best to make sure that I'm hydrated before I start training. That way, I don't need water as much and therefore won't be tempted to use that break to procrastinate.
Intensity in exercise selection makes a big difference. While this might lead you to think that I prefer higher intensity, low rep calisthenics, you'd be partially right. I've found that my training feels more complete if I throw in just a little bit of higher-rep exercises. I find that the combination of max strength and strength endurance is just a killer. When time is crunched, I like doing exercises that move as many joints as possible and move the body through different planes.
Generally, I've refrained from giving routines, I think that you could get some good use out of doing some of these routines. All of them should take you no more than 20minutes. As always, feel free to adapt them to your needs.
Jump Rope, 3 minutes
Hindu Squats, 150 reps
Jump Rope, 1 minute
Hindu push-ups, 75 reps
Jump Rope, 1 minute
Wrestlers Bridge, 3 minutes
50 8 count Bodybuilders or Burpees
Neck rotation exercises from headstand position, 10 reps each
Straight bridge, 1 minute
Ab wheel roll-outs, 10 reps
Neck rotation, 10 reps
Gym Bridge, 1.5 minutes
Ab Wheel roll-outs, 10 reps
Neck rotation exercises, 10 reps
Wrestlers bridge, 2 minutes
ab wheel roll-outs, 10 reps
Pistols, 5 reps each leg
Spider/T-plank Push-ups, 30 reps
Inchworms, 5 reps (you could sub the ab wheel)
pull-up/hanging leg raise combo, 8 reps
Repeat circuit 3 times