It can happen any time, and for any length of time.
We never know where it will happen,
and as they usually only last up to 1.5 hours thats what we've come to expect.
But over the last few weeks we had 2 that went for around 5 hours. Pretty annoying, hard for Soeun to get work done if its a sit-at-computer work day. As we're expecting it to come back on we just stay home and wait, if we'd know yesterdays was going to be so long, he could have taken laptop to a cafe in another part of town.
Most of Cambodia doesnt have mains power at all. Often people have a car battery at home to recharge their phone and power fans. But as we have mains power we rely on it so much- we have a fridge, washing machine etc etc.
When we lived next to the girls dorm a few years ago, I used to get annoyed when the power went off esp if i had been about to put on a load of washing. The girls next door however weren't worried, they didn't have a washing machine, I saw them squatting out the back just hand washing their clothes as normal, while I was waiting for the power to come back on before I could do mine.
My mother in law was laughing at me last week when she visited. For lunch I had been planning to take some leftover out of the freezer and defrost them in the microwave.....lunch plan thwarted due to power cut! Her preserved Chinese radish ("white carrot" and fermented fish paste, and freshwater shells from a cart on the street were all ready for her to eat, no microwave needed.