I agree w/ Jen... in our case Ayden was on a vent b/c of his lungs - no structure. The next thing is CPAP. The signs that lil G is ready, may not be completely obvious... ours was that Ayden wasn't sleeping. He has done this through out the weening off the rate to CPAP and then through weening off the vent too. It only took us 2 sleepless months to figure out that we were weening him too slow!!
Then Ayden actually decided that he was going to ween himself from a rate to CPAP in 4 nights!
Once he found out about the CPAP, he didn't want something telling HIM to breath. haha
So glad that you have been able to get him off the O2 - I wish we could have done that!!!
How fast - from a rate to CPAP, during the day took probably 2-3 months; nights like I said took 4 days. Then from the vent to off the vent during the day took about 5-6months and nights took another 3-4 months. All of which was done at home. Our pulm gave us some guidance, but for the most part, said to just go for it at the rate Ayden wanted to.
We did go into the office and try the initial trial at the office. He started to come off the vent at about 16mo of age.
wishing you the best of luck!!