A train an hour seems reasonable to me at this point. I think a train every 20 minutes would result in a lot of empty trains. Good to have extra capacity for the future though.
I really hope they don't wait decades to build the other half the line once Toronto-Quebec City is done. Windsor itself may not be big enough to drive major traffic, but I'd bet that if you put stops in Kitchener and London, those two would be very busy.