Millenium on 24 February 2012
If ever, which I'm not sure it will. No sooner than Q3 -Q4 2014.
Microsoft is not going to drop the Xbox 360 like they had to do with the Xbox, and will sell it like the PS2 in it's later (last) years for ~$99 and perhaps start selling it in countries which they currently aren't or can't yet.
And if it ever does manage, it won't matter anymore, next-gen will have started then.
