For upper body, the usual strength range is 6 reps.
For lower body, it varies wildly from individual to individual.
6 reps might work nicely for me on a squat, 20 might work better on a leg press.
If you're doing 15 reps and don't feel like it was much of a challenge when you have completed it, then you need to think about increasing the weight.
If you have a weight so heavy that you can barely do 3 reps, you may be totally missing your quads and working your hips/ass.
You need to experiment mate. Everybody is different.