The washing must continue until the impurity (its colour, its smell, and its taste) disappears (i.e., it loses its Najasa 'Ayniyya status), no matter how many washing it takes. So, for example, even if it took you 6 so-called washings to remove the impurity by which point the 'ayn of the impurity is removed, then this is considered to be your first washing. So you still have a remainder of 6 washings to perform.
If during the washing (and if you still have not added purifying earth to the affected area), parts of the impurity moves to another place due to the sprinkling or the splattering affect of water, then the new place must [Wajib] be treated with purifying earth. This is why it is preferable to mix the earth with water in the first washing and thereby treating and removing the 'ayn in the first washing.
If the washing is done in places where there is a lot of water (i.e., more than the qullatayn) and the water is not moving [ma' rakid; such as in a swimming pool], then it is sufficient to move the affected area 7 times back and forth (each movement whether back or forth counts as one). If one is submerged and keeps still, then that is considered one washing even if it lasts for a long time. If the washing is done in areas of running water [ma' jari; such as a sink], then it suffices to pass the water by the affected area 7 times. Of course, in all of the above situations, the rule of 'it counts only as one washing until the 'ayn disappears,' applies. So if the river is clear for example, then it would be necessary [Wajib] to apply purifying earth onto the affected area either by muddying the water (from the riverbed) or doing it before you enter the stream.
