Why the first year is the fastest
Untrained muscle is highly responsive to a new stimulus. In the first year of consistent resistance training a previously sedentary person may add a meaningful amount of lean mass, with several kilograms being a commonly cited range for men and somewhat less for women. This early surge is sometimes called the novice effect, driven by rapid neural adaptation alongside genuine hypertrophy.
As training age increases the muscle has already captured most of its easy adaptive headroom. Each subsequent year yields progressively less new tissue, so a second year might add roughly half of the first, and later years considerably less again.
Diminishing returns and individual ceilings
By several years of serious training many lifters are gaining well under a kilogram of new muscle per year and are working mainly to refine and maintain what they have. The exact figures vary widely with genetics, hormones, sleep, nutrition, age, and how a person started, so any single curve is an average rather than a personal forecast.
The practical takeaway is that slow progress in an experienced trainee is normal physiology, not a sign of doing something wrong. This is general educational information rather than a guaranteed timeline; individual results differ and a qualified coach or clinician can give personalized guidance.