A lot women think if they do too few reps at the gym - that is, lifting a heavier weight fewer times - they will get big giant muscles. But for most women, this is not a concern. Your body just isn't designed to put on big muscles, so it probably won't happen.
You don't have to do fifteen to twenty reps! It takes forever, and you'll get bored. If you find that you are spending a lot time at the gym, why not try using a heavier weight and doing fewer reps? It will pass the time faster and you'll get just as good results, if not better.