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)