This depends on your goal. If your goal is to decrease body fat, you should do resistance training as often as possible, with a caveat: do not train the same muscle groups on consecutive days. The muscles need at least 48 hours to recover and rebuild between workouts. However, you can work one or more groups of muscles one day and different groups of muscles on the following day. For example, one could work the muscles of the upper body on Monday, Wednesday, and Friday, and the muscles of the lower body on Tuesday and Thursday. While it is possible to overtrain, this is generally an issue of intensity as opposed to frequency. Numerous studies have shown that post exercise oxygen consumption — the rate at which the body burns calories at rest after exercise — lasts considerably longer after a resistance training workout than after a cardiovascular workout.