Depending on your fitness goal(s) you need to do the right amount of reps to the point of fatigue (when you can't perform further reps due to loss of strength and form), have the proper amount of rest between sets, and an adequate amount of recovery days before exercising the same muscle group(s) again.
Whether you're set on losing weight, getting more fit and toned, putting on some muscle mass, or increasing strength you need to avoid training in vain. Lift with the proper amount of weight to reach the proper amount of reps, perform your sets with the correct amount of rest, and avoid over-training or under-training during the week.
Below is the correct rep-range, rest-time between sets, and amount of recovery days needed for each fitness goal when it comes to lifting weights:
A common mistake is thinking that lifting as much weight as possible will make you look bigger. It makes sense to think like this but that's not how it works. If you're lifting heavy weights that means you're most likely doing 6 reps or less per set, which increases your overall neuromuscular strength NOT size. So follow these guidelines if increasing strength is your goal:
Reps (per set): <6 - Ideally 3 to 4
Rest (between sets): As much time as you need < 5 minutes
Recovery (days): 3 to 4
Lifting the heaviest weight in the gym will not make you bigger nor will doing as many reps as you can with lighter weights. To gain mass you need to find a happy medium. Ideally you want to aim for 10 reps to failure, 60 seconds of rest between sets, and a couple days between exercising the same muscle group(s) again. In general stay within these ranges:
Reps (per set): 8 to 12 - Ideally 10
Rest (between sets): 30 to 90 seconds - Ideally 60 seconds
Recovery (days): 2 to 3
MUSCLE TONING & WEIGHT LOSS
Losing weight and toning up is also known as muscular endurance. Technically this is anything above the 12 rep range but I always suggest 15 reps minimum for better results (especially if you're new to the gym). If tightening the body and trimming down is your aim then follow these three R's:
Reps (per set): > 12 - Ideally 15+
Rest (between sets): 30 to 60 seconds
Recovery (days): 1 to 2
DON'T TRAIN IN VAIN
If you're serious about reaching your fitness goals whatever they may be, do it right. Avoid training in vain by working out within the proper rep, rest, and recovery ranges. The above doesn't account for rep speed, percentage intensity per 1 rep max, or how many sets you should be doing but it's a must for your fitness toolbox.
Even if your goal is to increase strength or add muscle mass, I highly suggest starting off within the 15+ rep range for a period of time to build a solid foundation and body mind connection. This is especially important to do if you've been doing things wrong all along or you're just starting out.
If you're new to exercise start off by doing 15+ reps per muscle group, 1 to 2 sets, 2 to 3 times per week, for at least 4 weeks. Your reps should be done with proper form before increasing the weight being moved, which will reduce the amount of reps as it's heavier. Applying this will set up a foundation of muscular endurance that will benefit you long-term when you focus on different fitness goals.
Get Fit, Stay Fit, Enjoy Life!