5.30 How do I change Emacs’s idea of the TAB character’s length?

Set the default value of the variable tab-width. For example, to set TAB stops every 10 characters, insert the following in your init file (see How do I set up an init file properly?):

(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.