Blood Oxygen level question.

How long does it take for the blood oxygen level to normalize at 15000
ft, after one discovers the flow meter was not adjusted correctly?
Also at what altitude does Oxygen become beneficial, assuming an
average person? Are there any factors, other then altitude it self,
that can lower Blood oxygen level during flight assuming supplemental
oxygen is not used between 5000 and 10000 ft and the pilot has a normal
cardio vascular health? What oxygen level is considered optimum at sea
level and 10000ft to 15000ft?
Udo