It can take your mini fridge anywhere from three to 24 hours to get completely cold. For that reason, it's necessary to plug in the machine well before you want to use it. After plugging it in, use your user manual to decide what temperature setting it should be at. The manual might inform you of how long your specific model will take to cool. However, you can put a fridge thermometer or small bottle of water inside to test the temperature before you start loading in your perishable food.