I think most newbies can accomplish 21+ within a year to year and half of training. I know I gained over 20 lbs my first year and that was on a ****ty full body workout and not eating the best diet. That's with average to below average genetics.
Now putting into account above average or awesome genetics. You could see some one doing that in half the time. You know someone with full muscle bellies, naturally strong, more free flowing testosterone, big bone structure. Which is probably most of the guys in the nfl and most black people lmao
For intermediate and advanced people it will gradually taper down over the years. Some guys with over 5-10 years of training experience get ecstatic when gaining just 5-10 lbs in a whole year.
Again though 21 lbs of dry muscle mass is a lot. Yet 21 lbs of muscle+added glycogen is very attainable for most given they don't eat like pigeons and train using individualized complex movements and isolation movements in a well designed split.