According to the answer on the Straight Dope if a zombie apocalypse were to occur (one where it is spreading rapidly causing immediate chaos) this is how long you can expect your power to stay on:
Coal power (16% in Canada): as short as 2 hours (up to 18 hours if you're lucky)
Nuclear (12% in Canada): 500 days if no abnormalities or equipment failures occur (which is very unlikely) .. for those in Pickering it could be up to 700 days based on amount of fuel stored
Hydroelectric (60% in Canada): several weeks
Natural gas: 1-3 days
Since small issues come up constantly, they figure within 24 hours almost all the power would be out.
Keep that diesel tank topped up for your generator ;)
The Straight Dope: When the zombies take over, how long till the electricity fails?