Realistic muscle gain can vary widely depending on a variety of factors including an individual’s starting point (beginner vs. experienced), nutrition, training regimen, genetics, and more.
For beginners who are starting a new resistance training program, they may experience more rapid gains initially, sometimes referred to as “newbie gains”. These initial gains are often partly because of increased water content and glycogen storage in the muscles, not just muscle fiber growth. It’s not uncommon for beginners to see noticeable improvements in muscle size and strength in the first few months.
However, true muscle gain is generally a slow process. The general consensus is that under optimal conditions, one might be able to gain about 0.5-1 lb (0.2-0.45 kg) of muscle per week initially as a beginner, although this rate generally slows down as you become more experienced.
So, over a 24 week period, a beginner might potentially gain anywhere from 12-24 lbs (5.44-10.88 kg) of muscle, but this is somewhat the upper limit and a more realistic expectation might be slightly less than this, especially as individuals vary widely in their response to training. Also note, gaining muscle at this rate would generally require a very well-structured training and nutrition program.
As individuals progress past the beginner stage, muscle gains will slow considerably, and experienced lifters might find they are only able to gain a few pounds of muscle over the course of a year.
As always, it is recommended to consult with a healthcare provider or a fitness professional to create a plan that is best suited for your personal health goals and needs.