What is the ruling on doubting the ease of impurity for one who is not prone to obsessive doubts?

Whoever doubts whether a impurity is little or much, let him consider it little, because the default is that it has not reached the extent of muchness, and certainty is not removed by doubt. As for the one afflicted with [obsessive] whispers (waswas), he must ignore these whispers and not pay attention to them.