Bus from Kowloon to Shenzhen Airport--How much time should I allow?

I'm leaving on a domestic flight tomorrow (Sunday) at 20:35 and will be checking luggage. I would probably want to arrive a little over an hour before the flight to check in. What time should I take the Kowloon-SZ Airport Bus? My hotel in Kowloon suggested allowing 5 hours but that seems a little too cautious.