How much and how often you rest is important. You want to build endurance and that can't be done by tearing yourself down. Trumpet practice needs to be spaced out to give you ample time to rest before your next exercise. Don't wait until you are tired, rest between each exercise or section you practice. You want a method to keep you from getting impatient. By putting a Sand Timer on your music stand, after you've finished an exercise, you simply turn the Sand Timer over and it begins your 30 Second rest period. No batteries, no maintenance and it makes no noise.