Drupal - Drupal 6 end of life

I think the advice from Choosing a Drupal version is probably the best to follow:

At any given time, there are two major release series of Drupal which are supported. Currently, these are Drupal 6 and Drupal 7.

Essentially, official support for Drupal 6 will stop when Drupal 8 is officially released.

You can find further discussion on the So long, Drupal 5.x (End of Life Announcement)

While the official answers above are correct you need to look at the big picture. There is extremely little core work going on with Drupal 6 as it is tough to fix bugs without the extensive testing framework in place (and even with that, D7 bugfixes occassionally go haywire). Views is essentially on life support too. Obviously, quite a few contrib followed. There are only so many hours in the day and for a lot of people it's better spent on Drupal 7 (in the case of Views, Drupal 8 even).

So while it would not be impossible perhaps to gather a few large Drupal 6 sites and extend the support of D6 perhaps as a commercial venture (we considered this even for Drupal 5), but the problem for such a thing would be that you need to maintain everything: core and contributed modules, including the modules that were crappy four years ago and are without a maintainer. You get the picture.

The former security team lead (one of the very few who could actually pull this off) hints at such a commercial venture being planned at http://groups.drupal.org/node/291243#comment-907858. Edit: this one won't happen. Other, unknown entities of course might still pull it off but I have serious doubts.

It stops to be supported when Drupal 8 will have its first official release that is not an alpha release, a beta release, or a candidate release. The exact date of when that will happen is not known, but in the moment it happens, Drupal 6 will not be anymore supported.

This is exactly what happened with Drupal 5: When the first official release of Drupal 7 (excluding alpha, beta, and release candidates) was released, Drupal 5 stopped to be supported. So, in general, you can say Drupal X stops to be supported when Drupal X+2 is officially released.