I'm no fitness expert by any stretch, but I have several friends who are. They usually say that 5 reps/set is the magic number. If you can easily do more than that, you are probably ready to increase the weight. You should feel somewhat fatigued after a workout.
Don't get me wrong, you are certainly not hurting yourself and if you are losing weight then it's helping your burn the calories. But the reason you are not sore is because what you are doing is easy for your muscles, so there is no reason to tear down and rebuild.