Referal accepted going ahead with assessment for son. What next?

My son age 3 1/2 was referred by his preschool for ASD assessment.  Got the letter yesterday saying they agree to assess on info they have. Asigned  with a autism coordinator who will be in touch soon.

What happens next? How long does it take for assessment to start? And how long from assessment  for diagnosis? 

It's all changed since my 12 yr old was diagnosed five years ago. He wasn't  invited to an.assessment  as such. The reports from his school and agencies involved were sent to a panel  who made the desision. 

Thanks