Based on experience and recommendations from successful coaches, I’d say around 100 reps per week and muscle group is a good baseline to start from.
Or at least 100 reps per movement if skill is the aim.
You should keep your work between 80-120 reps per week and muscle group to get a training response. Anything less is probably not enough, and anything more is most likely too much. People can often be divided into fast responders and slow responders. Fast responders (those annoying dudes that build muscle easily) can stay between 80-100 reps, while slow responders need more work, 100-120 reps. Same thing can be said about different body parts. The pulling muscles (back, biceps ect) do often need more work than the pressing muscles (chest, triceps ect).
If you want to go for more a short period, you should always follow that with a period of less. Otherwise, you won’t get the supercompensation response you want. Instead, you risk overtraining.
How many repetitions you do also depends on the intensity of the work (how heavy the weights are) which depends on the quality response you want. The higher the intensity – the less reps you do. Here is an illustration of that principle.
The work can be distributed into different sessions during the week, and different exercises for the same muscle group. Most common is something between 3-5 different exercises for the same muscle. The repetitions per exercise are then normally divided into sets. How many sets depends, again, on the intensity. Same principle as mentioned above.
Lets say you’re aiming for hypertrophy. You need at least 90-100 reps per week which you can distribute into, lets say, 3 exercises for the target muscle. You can probably do these at one session. You can use an intensity that allows 8-12 reps per set. It means you can, for example, do 3 sets of 10 reps on one exercise, 4 sets of 8 on another and maybe 3 sets of 12 on a third exercise. That will give you 98 reps in total. Plenty enough, but maybe room for one more exercise every other week to give it an extra kick (it’s common to use a “finisher” when training for hypertrophy).
Now, this is not written in stone of course. People tend to respond a little differently. But it’s a good place to start, it works for most people, and gives you an idea of how to think when planning your training. It’s all about the principles. If you feel stuck in your tracks, well, compare your work with these principles and adjust your program accordingly.
Baechle, Thomas R. & Earle, Roger W. (2008) Essentials of strength training and conditioning, National Strength and Conditioning Association, Human Kinetics.
Christian Thibaudeu (2014) The Black Book of Training Secrets: Enhanced Edition, Createspace.