@xXSeraphimXx your questions are WAY too vague.
When you say ‘add an inch in a month’ are you talking about strictly lean mass, or does fat count too? I bet I could eat my way to an inch of pure size in a month.
You’re also not addressing training age of the individual. That matters a LOT regarding all your questions. And also how small the individual is starting out.
So when I started lifting, I was underweight. I was 5’11, 125 lbs, at 18 years old. So when I started lifting and eating to grow, I probably put on muscle at a rate that very few people can exceed naturally. It happened very quickly. But all the factors above are why it happened. It wouldn’t surprise me if I had a couple months in a row that I added an inch to my arms. I put on 20+ lbs in the first few months of lifting, and it was mostly lean mass. I looked like I’d used steroids. But my body was just primed to grow from being so incredibly underweight, and in my best years.
If i were to put on 2 lbs of muscle over the next 4-6 months right now, I would be very happy. And that would be with ‘assistance’ It gets harder and harder to do as you progress. Adding a lean inch to my arms in a month is absolutely not going to happen ever again in my life.