Sonic, you've got things very mixed up I'm afraid with all the numbers higgly piggly (obviously you are upset)
Though high concentrations of 02 are not good for you (they are toxic to the lungs) they can be used for as long as needed to try and maintain an adequate amount of o2 in the blood (and hence your organs)
What the Drs probably mean is that the fact your friend NEEDS so much o2 to try and achieve this implies his lungs are in a bad way and perhaps not improving. I think they are trying to hint that your mate is not going to get better and may well die (sorry to be blunt)
But there is no specified time that high concs of 02 can be administered for, I hope that makes sense?
The real question is how sick is he?
How much support is he needing for his lungs (how much pressure, how much 02 etc)
Have any other organs failed?
What was he like before? ie runs 5 miles everyday or can't climb a flight of steps?
If they say he is requiring loads of support for his lungs and then talk about switching the ventilator off then I'm afraid what they mean is withdrawal of treatment with death as a consequance.
Having spent more years than I would care to mention on ITUs I can tell you that sometimes we try for too long on futile cases and sometimes we pull out a little too early ( ie someone has 99% chance of death so pull out, but 1% will be the miracles that survive. If it was me I'd quite like even that 1% chance)
All the best, feel free to pm me and I'll try and help