clickclick - the higher you are, the more oxygen you use, even when breathing it through an (allegedly!) airtight mask. I can't remember the medical explanation behind it all - something about the body "exhaling" oxy through the pores as well as the mouth and nose, so the exhalation rate is higher at altitude due to oxygen's decreased partial pressure - but the short answer to your query is that if you maintain F180 for longer than the book says, then you (or your pax) will run out of oxygen quicker than the book says.
As someone else mentioned above, it is the operator's responsibility, at the flight planning stage, to ensure that the route allows the oxygen descent profile specified by the manufacturer to be met. If it can't be done, then another route must be flown. Often a specific escape route has to be constructed, and the crew should be aware at all times of were they're going to go in the case of depressurisation - that's what they're paid for.