Having a day that he was comfortable is a wonderful blessing for you both.
O2 is fine now as a palliative measure and to be honest if his sats go below about 92 OR if he shows any air hunger symptoms you can just give it to him. Once again it's comfort and comfort doesn't always match numbers so again go with your gut feeling.
As to how long this can go on, I'm afraid it's cruel indeed when it drags this way. It's hard to say how long, but I think you know that it can't be a very long time. It must be exhausting for you - the emotional and mental preparation you would be doing all the time.
So I'm sending more hugs and strength and love your way. Hating ALS along with you, but loving what an amazing job you are doing. Inside there Cliff knows this too.