If you are low on time, here are some suggestions that can help you out when you are low on time.
1. If you are late to class, just go. Even if you are late, something is better than nothing. Just make sure that you warm up for at least 5 minutes. Jumping jacks, crunches, push ups, and squats are good to do with bodyweight alone to get your blood flowing. Every minute counts. You should try to roll and do the drills with the least amount of rest as possible, since you don't have that much time to train.
2. If you can't make it to class, then try working out at a gym, park, or at home. Try to do complex exercises to workout a lot of muscles at the same time in order to cut your workout time. A couple of exercises that you can do are thrusters, deadlifts with sprawls, burpees, and clean and press, or you can combine exercises in a circuit format or in a super set to increase the intensity of the workout. Try to rest from 20 seconds to 1 a minute between sets to help out on your cardio.
Sometimes when I can't go to class or the gym, I workout at home. Five or Ten minute workouts are it for me. I don't care if you are doing Insanity, P90X, or any of these workouts given below. Something is better than nothing. Sometimes the shorter workouts are better than the longer workouts, because they allow you recover faster between workout sessions.
5 Minute Workouts
10 Minute Workouts
I remember a lot of times when Eddie Bravo talks about stretching at anytime to increase your flexibility. If you are watching tv or sitting down and hanging out, it's a great time to stretch or put yourself in a position to stretch. When I'm in class and the instructor is going over a move, I stretch. I try to make the most of each time, even when I have time.
Do want you can and do what you like, but no matter what, you are becoming better than what you were before. When you train, you can only get better. Whether, it's 5 minutes or 2 hours, you are putting in work.