Your body is constantly rebuilding and breaking down muscle tissue. Remember that protein and muscle is what your body is comprised of. Not just your skeletal muscles. You gain muscle when the rate of muscle protein synthesis is greater then protein breakdown. Scientifically it means you are in a positive nitrogen balance because you are retaining more nitrogen then your body is excreting.
Building muscle is a slow process. Naturally you can gain about 10 pounds of muscle per year. Enhanced it would be 20-30 pounds per year. Keep in mind that it's very individual and the longer you have been training, the harder it will be to build additional muscle as you are closer and closer to your genetic potential.