Try this, you can pass a date
var date = new Date();
date.setTime(date.getTime() + (3 * 24 * 60 * 60 * 1000));
$.cookie("the_cookie", "the_value", { expires: date });
or send the days as parameters
$.cookie('the_cookie', 'the_value', { expires: 3 });
Output
if you run the function in browser console where jquery cookie is already present as source, in both the cases it prints
"the_cookie=the_value; expires=Mon, 30 Dec 2013 08:28:36 GMT"
(current time is Mon, 27 Dec 2013 08:28:36 GMT)