I know this is horrible and wrong but, just out of curiosity, I Googled "smoking Everest". today. I could lie to you and say that I was looking for advice about how far in advance I need to quit but the honest truth is I was looking to find out what would happen if I did this and DIDN'T quit.
So, some interesting results. What came back is that smokers adapt BETTER than non-smokers to altitude. This is for various technical reasons but it basically boils down to the fact that our bodies are used to be oxygen deprived and therefore our haemoglobin releases oxygen faster to the body.
Now don't get me wrong, I'm not planning to puff my way up the bloody thing like the old-timers did but it might be an idea to quit just before I do this trek.
Something to think about I guess.