I don't think the Plugin supports any number of character for marking.
But according to the Plugin site -
MASKED INPUT PLUGIN[
^]
Quote:
You can have part of your mask be optional. Anything listed after '?' within the mask is considered optional user input. The common example for this is phone number + optional extension.
jQuery(function($){
$("#phone").mask("(999) 999-9999? x99999");
});
So, you can put a "?" mark after 7 characters, it would work.