Ed,
How many hours do you get out of a refill with two people?
Depends the data is out there. Depends on
Size of bottle
Altitude
O2 flow
# of people
Type of mask.
You are not on 100% O2 unless in the flight levels.
A mask that captures some of your exhale is more efficient, better 02 economy.
Cannula (small tubes that sit in your nostrils) are efficient but they are continuously flowing 02. If you are smart you flow only enough to maintain proper blood O2 saturation. A fairly cheap clip on finger blood O2 meter ($30 to $40) will do the trick. You can waste O2 or conservative. BTW those blood O2 meters are good even if not flying at altitudes that require supplemental O2.
The rocky mountain pulse demand device, research it. They're expensive and some people say it doesn't save you much. It depends on many factors if it pays for itself. How much you use it is one factor. If you live in Colorado. Wyoming, Utah, fly over mountains (Rockies, Sierra Nevada, Cascades) then it will save O2. You live on flat parts of USA you don't need O2, waste of money.
The issue is we tend to want small O2 bottles for space and weight. To refill can be easy or hard, cheap or expensive depending where you live. Me and several other pilots went in on a very large commercial O2 bottle (leased) and filled it. Sits in hanger. We split the cost and refill our own bottles fairly cheap. Also don't forget hydrostatic pressure test. If you have a tiny bottle on long distance trip you will be refilling at FBO's, typically $75 flat rate small or larger bottle, same price.
That is why I started thread on O2 generators. Unless terrain or weather requires it, or winds are favorable in a normal aspirated RV flying below 12,000ft, say between 8,500 and 11,500 is good enough. If non smoker good health you should be OK. However your night vision is affected at 8,000 ft, so supplemental O2 is good. Again, like most pilots rarely are flying over 10K ft , most pilots rarely fly at night extensively, or at least night and high attitude. For those that do, O2 is wise even if not FAR required.
TURN KEY O2 KITS FOR PILOTS. With a little effort you can buy used gear, bottles, fittings, regulator, flow gauge, mask for much less rolling your own... No fancy pulse demand gizmo needed. If you buy a total system get it at airshow price. There is a bit of a markup. With that said it's a huge convenience just to buy it all at once from one place. Just expect to pay a lot more.