Not only are the bacteria fairly resilient, they're also really quick to reproduce in the filter. Your bacterial doubling time is in the ball park of 8-10 minutes, under ideal circumstances. So, even if you assume that circumstances suck, and it's 15 minutes, and you lost 99.9% of the bacteria... in 15 minutes, you have 0.2%, in 30 minutes, 0.4%, in an hour, 1.6%... in two hours, you're back up to 25% of capacity. In 2.5 hours, you're back at 100% capacity.
The big concern with a canister filter is not the loss of bacteria, which I use collectively to include all appropriate microbes, but the loss of oxygen. When you lose flow, all that gunk continues to rot, and the oxygen levels go down (without fresh water and oxygen entering the unit). It eventually goes anaerobic, and you begin to produce H2Stinky. When the filter resumes function, all that anaerobic gunk flushes into the tank, and not only makes for a really pleasant smell in the house, but can kill your fish. (Technically, it can kill you, too, but the volume in your filter is much too small, unless you were in a very confined space...). This can happen any time you lose power to your filter, even if you just kick the cord. And it can happen pretty quick -- in an hour or so.
The amount of time and amount of sulphurous gas released are going to be a function of the amount of gunk in the filter. If you haven't cleaned it in a while, there's more to decompose, it happens faster, and it is worse -- if, as you did, it's relatively clean, you probably don't really have to worry.
At the very least, I would disconnect the power to the canister filter so that it doesn't come on when the power comes on, and I would then dump the water out of it and refill it once the power returns.
As an aside, as long as the bacteria remain "moist," they'll be fine -- so you can dump the water out for 15 hours and keep it closed.
By the way, whenever power is out, I unplug all of my filters and pumps anyhow. This serves two purposes. One, a lot of the time the power will flicker on and off and on and off and on and off before it comes on and stays on. This can be hard on your motors (as well as your electronics -- it'll burn up a computer). Secondly, sometimes things don't start properly, such as a pump that needs priming, or an impeller that's sticky, and starting them manually, one at a time, gives me the opportunity to make sure that everything is going without just "taking it for granted."
