I received the pre-arrival letter after 14months. Spouse visa here and june 2017 applicant.Pre arrival letter is sent to the applicant once the sponsor has been approved and eligibility is passed. This takes place around the time your file is transferred to your local Visa Office.
Pre arrival are free services provided to new/landed immigrants. There are a lot of services provided to new immigrants which is really nice gesture from the Canadian government.
Typically after pre arrival your background check and security check start as these steps are completed by the Visa Office