There is no currently defined end of Ubuntu (new versions are planned to be released and no mention of this regular cadence of releases stopping).
How long Ask Ubuntu lasts depends on 3 factors: 

 1. How long Canonical allow us to.
 2. How long Stack Exhange allows us to.
 3. How long we continue as a community.

The actual information we post to Ask Ubuntu is under a [Creative Commons licence][1], can be freely copied (with attribution), which in the event of any combination of the above 3 points will allow someone to preserve the information elsewhere and even setup an alternative to askUbuntu.

  [1]: http://blog.stackoverflow.com/2009/06/attribution-required/