How long does it take European (UK) nationals to clear immigration in Toronto typically?

I am arriving from London (bus class if that makes a difference) on Air Canada at 3:05pm on a Monday. I am travelling on to Sarnia. I could either get a car (taxi) or fly. Flight to Sarnia departs at 18:40. If immigration & baggage will take <~1 hour it makes sense to get a car to Sarnia. Otherwise I would fly. Price is about the same. Any thoughts?