Yeah they can go a long time. I've never had to deal with more than a 14 hour power outage most of ours are in winter and the power companies really have to get it back on before we all freeze to death in these WI winters. Its like negative outside right now, cuz of that blizzard last night. Yeah we got slammed. A foot of snow here. That we can deal with, its the wind. My classes we canceled 20min before the started. So of course I had already dug the SUV out from under a foot of snow and drove 10 miles. They could plow all they want, the wind just kept putting it back on the roads. I went threw some ~18" drifts. 5 minuets later on my way back home you couldn't tell I had gone threw them at all.
In canister it is still a good idea to circulate the water. Usually at the 8 hour mark I disconnect the output hose from the tank, lowering it into a bucket creates a instant syphon. Reconnect the hose once the bucket is full, then I pour the bucket back into the tank.
If I was expecting a super long outage like 5 days I would either throw the media in the tank, or completely remove it and put it in a bucket and feed it ammonia. My tanks are densly planted so I can get away with remove the filters completely if I don't feed the fish.
Main thing at my house is we have a well. Wells need electricity to work
. Water changes are not an option. This is perhaps even more annoying than lack of power. Once the tank runs out, basically that uses water works. You go to wash your hand............... no water...... this gets incredible frustrating. You keep turn on the facet without even thinking about it. A couple hours without power makes you realize how much you use water forget electricity.