This depends on a lot of factors. Let us start off by saying that it’s important to know that relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature. When the air can’t “hold” all the moisture, then it condenses as dew. The RH that can be reached depends on the room temperature, usage and room size. The amount of moisture that can be released in the air (per hour) is and 220 ml. 1 ml is equal to 1 gram.
For example, at 20˚C a cubic meter of air can hold a maximum of 18 grams of water. At 25 ˚C, it can hold 22 grams of water. If the temperature is 25˚C and a cubic meter of air contains 22 grams of water, then the relative humidity is 100%. If it contains 11 grams of water, the relative humidity is 50%. So if a room is 4 x 2 x 2,5 meters (20 cubic meters) at a temperature of 25 ˚C, the air can hold a maximum of 440 grams of water in the air. Depending on the current humidity in that room, it should take just a few hours to reach the desired RH.