It went out somewhere around 11 and I think it was off for 3-4 hours. The last few times I immediately jumped up and got the process going. I read about how quickly things can go south in the tank. However, since the weather has been so mild, I figured the house temp would not drop too fast. If it had been still out when I got up, I would have cranked it up and gone to work. I would have come back home around lunch to check on things and filled it back up. I think it can run about 8 hours on a full tank.
Since they ship corals around the world in tiny bags, I figure they can do okay without water flow for day or so, but not sure about the temp. if it gets really cold.
So, it most often will serve as a piece of mind knowing that it is there is it there when needed.....however, for most people the likelihood of losing power for more than a day is probably much lower, but the impact (cost form loses) is still high. I would hedge that bet with a $300-$500 generator if I had a tank full of expensive SPS (especially if it is grown out which can take years).....also nice to keep that refrigerator/stove going.