Sorry, I should have clarified.
Unless you carry a kevlar-reinforced bottle with 6000psi of O2 and weighing enough to seriously affect your W&B
there is always an issue of O2 consumption, because refills are so hard to arrange.
One could argue that typical GA flights are only an hour or two and while that itself would make this a non-issue (4 pax at FL180 for 2hrs is OK from a reasonably small bottle) the problem is that the infrastructure for non-pressurised O2 refilling just doesn't exist around Europe. Occassionally one can get a refill from a bizjet maintenance facility, but one has to carry a range of adaptors and a lot will refuse to refill a portable bottle because "their insurance won't cover it". It's always pricey, at 50-100 quid a time. Plus the long wait, huge hassle if abroad.
So, on a typical away flight, one tries to make do with one initial fill, done back home at a friendly scuba shop (and a lot of those are staffed by anally retentive idiots who won't do it, which is why I got my own bottle in the end).
So one may have to be a bit economical with the stuff. With 2 people total, one can do a number of hours at FL140 with a normal sized bottle and cannulas. At FL140 I would use it all the time. But at say FL100, in perfect VMC, there is a case for either turning the stuff down (to say an "8000ft" flow rate) or not using it at all and turning it back on for the last hour or two to freshen up.
The thing is that FL140/160 is needed only over mountains, so that is another aspect of flight planning. There is just no way to do 10hrs with 4 POB at FL140 with anything that's remotely portable. Unless, perhaps, one spends $1000s on the new electronic control kit.
Only the most unfit people, or smokers, will have trouble at FL100. Above that it varies a great deal. One has to carry an O2 monitor; the thing you stick a finger into