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.