How long will food last in a refrigerator that is turned off

[Edited] I think 10 hours would be safe enough for most food: the first couple of hours the fridge will still be quite cold, and after that it takes more than a few ours for most food to spoil. When I leave milk out of the fridge for three hours at room temperature (say, 20 °C), nothing happens. I do this often. And the temperature inside the fridge will never reach room temperature in those 10 hours. When it is really hot in summer, it might spoil after 2 hours outside the fridge, but otherwise it takes much longer.

Cheese, food in jars, sauces, leftover cooked food: none of these are more sensitive to spoiling than milk. Fresh herbs are probably equally sensitive. Eggs are much less sensitive. The most sensitive kinds of food I can think of are probably raw meat and raw fish without preservatives. Smoked or salted meat and fish do not count: they can be kept much longer. I think even raw meat and fish will pull through, but you might want to apply the following tips to be sure.

Tip 1: put as much material in your fridge as possible, a day before the outage. That will "absorb the cold" that your fridge feeds it while power is still on. Then, once power goes out, the "cold reserve" in your fridge will be significantly increased, so that the average temperature of the interior will stay colder for a much longer time. The higher the volumetric heat capacity, the better. See this table ('volumetric heat capacity') for the best material; water probably wins. So stuffing bags of water in every nook and cranny would seem like a good idea. Or else bottles.

Tip 2: if you have a freezer, stuff it with bags and/or bottles of water, and put most of those in the fridge once power goes out (swap with some of the bags/bottles that were in the fridge). The freezer will probably do better, because it has much better insulation than the fridge. And put the most sensitive foods in the freezer maybe a few hours after power goes out (or maybe later—lest they freeze after all).

Actually, start putting as much water bags/bottles in the freezer as you can several hours before power goes out, and move them to the fridge once frozen. They will remain half-frozen for hours and hours in the fridge, so they will add extra coolness to the fridge. This way you're "saving up" cold from the freezer and increasing your cold reserve even more.

Tip 3: most refrigerators have a temperature slide/wheel/thing, which determines what temperature the thermostat aims at. You could set the slide to maximum power (so lowest temperature) a few hours before the power outage. Most people, including me, have the slide somewhere in the middle normally. The slide usually doesn't mark the exact temperature, but rather some symbols. It is usually found inside the refrigerator.

Tip 4: cook and/or eat any raw fish and meat before the outage. I think it would not be necessary at all if you follow the above tips, but cooking your meat now may just be simpler. Then either store the cooked meat or eat it. Incidentally, I wouldn't half-cook any meat: that may actually increase the rate of bacterial growth if it doesn't kill the bacteria completely. But something like salmon can be (fully) cooked first, reheated later.