Assuming you are talking about a ceiling fan:
It depends on the size of the room and the fan.
For a standard sized room, it can take anywhere from 15 minutes to an hour for a ceiling fan to cool the room down significantly.
The ceiling fan will help to circulate the air in the room and create a breeze. This will help to evaporate the sweat on your skin and make you feel cooler.
The fans can also help to cool the room down by lowering the air temperature. This is because when the blades of the fan rotate, they push the air downwards. This causes the air to compress and become warmer.
It is important to remember that ceiling fans do not actually lower the temperature of the room, they just make it feel cooler. So, if you are looking to cool down a room quickly, you may want to consider other options such as air conditioning.