I'd leave Toronto a good four hours before your flight. And for the airport in Buffalo, you may well be better off choosing the Queenston-Lewiston bridge rather than the Peace Bridge -- although I've waited 20 minutes there before! (But that was on Memorial Day -- presumably lots of people were driving home like we were.) MapQuest also recommends Queenston-Lewiston.

You may find the drive is under two hours, but making allowances for border delays and the like you'll probably be glad you planned 2.5 hours for driving and 1.5 hours for checkin. If you leave Toronto three hours before your flight, you'll probably make it, but you'll be biting your nails as you approach the bridge...

The best thing to do is to find some way (wireless internet, a friend with internet access, or the Niagara Falls Bridge Commission on 1-800-715-6722) to check waiting times on both bridges as you approach the divergence of QEW and 405, and just head for the less crowded bridge. Don't rule out the Rainbow Bridge (in downtown Niagara Falls) if the others are nasty, although that shouldn't be a primary choice.