Whatever the traffic situation is going to be is going to be a big factor. If you are taking 417 back, there probably won't be a great deal of traffic until you hit the airport property. From what I remember in December, there was quite a bit of construction going on around the south exit/entrance to the airport. This could cause a backup.
In any event you want to make certain that you arrive at least two hours before your flight, which brings you down to 6:45. Allowing 20-30 minutes drive time to airport...that takes you to 6:15. Add another half hour to be safe for checking the car back in I'd say you better be on the road by 5:45 P.M.
I know that sounds terrible having to leave three hours before your flight, but that is the time we live in nowadays.
Brian