1) It depends. I've spent 20 minutes and I've spent 1.5 hours.
2) You will have to clear US Customs and Immigration in YYZ. No *G does not help. Your biggest hurdle is the length of the line so US passport doesn't really help either. 90 minutes is probably the bare minimum to avoid risk.
3) With 6 hours your only real option is to rent a car. I'll defer to others on suggestions on travel. YYZ is very far removed from the downtown core. You're looking at least a 30 minute drive one way and, with your flight at 6:30, you risk getting stuck in rush hour.
In all honesty, with six hours, you might be better to hang out at the airport.
Out of those six hours you should assume you lose two to three to Canadian and US Customs and just getting around. That leaves you three to four hours to get a rental car and drive into the city. . . some time but not much.
Now that I think about it this is probably enough time to go to the CN Tower which is North America's tallest freestanding structure.