The way you count days of nida [seeing blood in normal time] and ziva [seeing blood in not normal time] is a point of disagreement between the Rishonim against the Rambam. The Rishonim hold seven of nida and then if more than seven then ziva. But the Rambam has this sort of approach which seems impossible to stick to. In his approach days of nida start when the girl first sees blood and then continues according to the order 7-11,7-11, 7-11, etc. [So if she see one day and then sees on day 30. Well to the Rambam that is ziva.] So let's say you have a girl 18 years old who clearly has not been keeping track. So any blood she sees could easily be ziva. [And even if she has tried to keep track,- well so what? With five colors of blood that are unclean and five that are clean  who can tell when she actually saw something unclean?

What I think is this: the best thing is to go with the simple approach of the Rishonim (e.g. Ramban/Nahmanides) that when she sees that is the beginning of nida. Then go to a river or sea on the seventh day and then at night she is clean. [Only in the rare case of seeing for more than seven days does the issue of Ziva come into play. Then if she sees for three consecutive days that is a zava. Then she would need to count seven clean days, and find a spring.