What is the suggested amount of time to rest muscle groups between workouts to prevent injury?
I want to tone (improve shapeliness of existing muscle) my body and workout on average 6 times/wk lifting weights, doing high intensity aerobic classes and doing the elliptical. My typical regimen at the gym involves 45 min of cardio and 6 exercises (15 reps) - 3 exercises of large muscle group (ie chest) and 3 exercises of small muscle group (ie triceps).
If you're working the same muscles each day, you're likely exercising far too much. Typically, a person should only work a muscle every 2-3 days. Check out the chart in this answer to "Is it healthy to exercise a muscle when it's still sore?" and a similar chart as figure 2 in this answer to "Importance of Rest Days". The optimum time to work a muscle out again is at the peak of it's supercompensation/adaptation. Working out more often inhibits any potential gains with fatigue, and working out too seldom causes the gains to already be lost by the next workout.
This is very individualized, and you need to find out what works best for your body. If you find that you are not progressing though, you need to change something up.
I believe you only need rest days if your working out big compound exercises. Before I started properly squatting and deadlifting and following a program I needed any hardly any rest days. If you are doing isolation machine exercises it definitely doesn't hit you as hard.
