If you're cooking the white rice on the stove in a pot of water it takes around 15 to 18 minutes.
Put the rice in the pot and then cover the rice completely with water and then turn the stove on high to bring the water to a boil and after it's boiling turn down the stove so that the rice can simmer and stir the rice a few times before covering the pot of rice with a lid.
Let the rice in the pot continue to simmer for the rest of the time which is between 15 to 18 minutes and the water should be absorbed by the rice when it's done.
Cooking rice in the microwave takes around the same amount of time but cooking the rice on the stove makes for better rice.