You should rest 48 hours at the minimum. If you only waited 24 hours, that would mean you would be working the same muscle group again the next day which you don't want to do. Depending on the intensity of your workout, your age and your ability to recover, you may need to rest as much as 72 hours. I work HARD--heavy weights and many sets. My real soreness usually does not set in until 48 hours later so I usually end up resting a muscle group for 72 hours before I hit it again. If you do not allow adequate recovery, you will not allow the muscle to rebuild itself before you go breakin it down again and you will subsequently become weaker and set yourself up for an injury. Don't feel like a wimp by resting a muscle group for 48-72 hours--it means you worked hard and need to time to recuperate!