I use a portable system with oxy-saver cannulas.
They recommend .3 litres/min at 10,000' rising to 0.6 at 18,000'. Above that you will need masks and the consumption rate rises considerably.
My 9 cu ft bottle gives 9-10 man hours at 10-12,000' at the low setting and keeps my saturation up in the 95-96% region.
1 cu.ft is 28.3 litres, so a few quick sums should give you the answer.
The dive shop in San Pedro does a refill for $15. A lot cheaper than the local FBOs.