Goal: Strengthen Your Core
If you want to strengthen your muscles, it is not a good idea to restrict your rest time too much. You need to allow your muscles the time they require to recover their strength completely. Heavily working a muscle that is not fully recovered is counterproductive. But you should not totally relax and fall asleep during your workout, either. A good average for a rest break is 45 seconds to 1 minute. But a rest break lasting more than 2 minutes between sets is too long.
Goal: Lose Inches Off Your Waist
Rest breaks between sets should be relatively brief: no longer than 30 seconds. A good strategy is to reduce your rest time progressively with each workout while striving to maintain (or even increase) your repetitions. For example, if you have done a workout with 30 seconds of rest time between sets, try to repeat that effort while taking only 25 seconds of rest. If, after several sets, you cannot keep up that pace, then increase your rest time to 30 seconds. During the next workout, try to do even more sets (or possibly the whole workout) with only 25 seconds of rest between sets.
Goal: Get a Cardio Workout Using Abdominal and Core Exercises
The ideal method here is to do circuits. That is, do different exercises one after the other with no real rest time in between. The only respite is during the transition from one exercise to the next. Throughout the workout, as the circuits become more and more difficult to accomplish, you can give yourself a 10-second break between each exercise.
Goal: Improve Your Athletic Performance
You should calculate your rest time based on the requirements for your sport. Thus, in sports with brief but intense explosive movements, follow the guidelines previously given for strengthening your core. In sports requiring both endurance and explosiveness, such as team sports, follow the guidelines previously given for losing inches off your waist. For pure endurance sports, follow the cardio model.