Realistic growth rates
Beginners can often gain noticeable muscle within the first few months of consistent training. Experienced lifters typically see far slower progress, sometimes just a kilogram or two of genuine muscle per year.
Why growth slows over time
Your body has a natural ceiling for muscle mass shaped largely by genetics, and the closer you get to it, the harder each additional bit of muscle is to earn. This is normal, not a sign that something's wrong with your training.
What actually drives growth
Progressive overload, adequate protein, and enough recovery time remain the three biggest levers regardless of experience level. Tracking your One Rep Max or FFMI periodically is a useful way to confirm progress is still happening.