In this book, Mind and Muscle by Blair Whitmarsh, PhD he states that for wll trained male athletes the maximum amt of muscle you can gain in a month is approx 1.5 lbs, or 18 lbs per year. This is naturally. For untrained men the first year can be up to 25 lbs.
Female athletes can gain 8 to 12 lbs per year. This is also done naturally via diet. Note the word, "athlete" typically someone who lives and breathes for their sport not a recreational bodybuilder or gym goer.