52:  How do I change Emacs's idea of the tab character's length?

  Set the variable default-tab-width.  For example, to set tab stops every
  10 characters, insert the following in your .emacs file:

    (setq default-tab-width 10)
 
  Do not confuse variable tab-width with variable tab-stop-list.  The
  former is used for the display of literal tab characters.  The latter
  controls what characters are inserted when you press the TAB character in
  certain modes.