ALS progresses differently in different people. However, a 2022 study found that a type of limb onset disease in the legs called flail leg syndrome took an average of 2 years to spread from the legs to other areas.
The study authors acknowledged that this was slower than most people who have low limb onset ALS. In turn, a 2023 review suggested that limb onset ALS has a slower progression to other symptoms than other ALS types.