When the ground materials become saturated with enough water they will start flowing. It's speed can range from being barely noticeable to rapid movement. The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be. Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.
An earthflow may affect as few as several square meters and up to several hectares.