I have a degree in exercise physiology, and for a while I was a personal trainer. Here are some notes I wrote years ago to help answer a question I got a lot about weight training: How much weight should I lift?
First off, it depends on what change you want to induce in the muscles. Lifting for 15-20 reps will primarily cause muscular endurance. Lifting for 8-12 reps will primarily cause muscular hypertrophy (making the muscles bigger). Lifting for 2-6 reps will primarily increase muscular strength. I think if you are going to lift you should lift in all three ranges.
How much you need to lit to be in each range is something you have to figure out for yourself. Pick a light weight that you know you can handle, and see how many times you can lift it. If you pick a weight that is really easy, you can always add more for another set. If you start out too heavy, you could put yourself out of commission for a couple of days.
I cannnot tell by looking at someone how much they should lift for a particular movement in any of the three ranges. If I said something like, “You should bench press 200 pounds for hypertrophy” without seeing how much you can lift or how well you can control that much weight, then that would be irresponsible.