suppose you come to the gym but can't spend an hour or so that day or suppose you want to chock your muscles with something diffrent,if that's the case,then you can try this,pick one exercise for each muscle group that you are suppose to exercise that day,after going through your warm up sets,do6 sets on each exercise to failure,that's it,believe me you will still get a good workout and your muscles will benefit.
advanced people can do more sets.