Remember that many people report a few months of gains followed by quite a few months of "stall" or even a give-back of some of the early gains.
After that, progress resumes but at a slower pace than the first few months. People often get most of their eventual gains in the first year or two.
I got almost all of mine by 1.75 yrs of detox.
People who are disabled or have MCS could still get additional gains after 3 or even 5 years. Detox seems to be rough when MCS is involved. My impression is that adult autistics also have a long road.