Is $800 dollars enough for one week in Canada (plane ticket and hotel paid for)?
Since you state that your hotel and airfare are paid for I am assuming that the $800 is for meals and expenses while in Canada. If so then yes, you should not have any difficulty surviving on that amount for 1 week assuming you do not get carried away. If you plan on a lot of activities and souveniers then you may want to bring some additional funds but if you plan on keeping things simple you should not have a problem.
If you mean that your hotel and plane ticket are already paid for, and the $800 is for anything else you'll spend money on, then it's hard to say if it's enough unless we know what you plan to spend money on. We have no idea what you think you will be doing in Canada.
Depends what you're doing / what it needs to cover
Well that depends a lot on you. I can easily spend $1000 in a day. I can also make $500 last two weeks.
You mentioned the accommodations are already paid for so a little over $100 a day for spending money is enough for simple dining, and transportation ( fast food/grocery shopping and public transit ) and entrance fees for tourist spots. If you plan on fancier restaurants, going out for alcoholic drinks, taking cab rides, etc., you can easily blow through your spending money before the end of the week.
It depends on where you go, and what you do (and eat), but, you should be fine if you don't go crazy!