Children with autism need intervention over a long period of time
Nine out of 10 preschool children with autism still have major difficulties within the autism field at school age, despite having received early intervention. A majority of the parents stated that the children do not receive sufficient support at school. New research has explored these issues in a study where children who had received an autism diagnosis between the ages of 2 and 4.5 years of age were followed up after 8 years. Ref. Source 3x.
I would think they would need a longer time. Autism is a lifelong challenge so they will need to be helped for the rest of their lives. The difference, as I understand it, is they will need less help in their older years if they have early intervention.
Parent-mediated therapy may help babies at risk of developing autism
The earliest autism intervention study in the world that uses video to provide feedback to parents of babies at family risk of autism, has indicated a reduction in the severity of emerging signs of autism. Ref. Source 3k.