Water from the oceans evaporates, condenses into clouds and freezes, and then it snows or rains, soaking the soil, filling the rivers, and returns to the sea. Water is always moving, renewing itself. This constant movement is called the geological water cycle.
Every drop, every water molecule has its own story to tell but, statistically, a molecule of water evaporated in the atmosphere re-precipitates after a week and a half and, once having reached a river, takes, on average, two weeks to reach the ocean or a lake. This average period of time that a water molecule remains in the atmosphere, in living beings, glaciers, etc., is known as the residence time.