If you're using jQuery Cookie (https://plugins.jquery.com/cookie/), you can use decimal point or fractions.
As one day is 1, one minute would be 1 / 1440 (there's 1440 minutes in a day).
So 30 minutes is 30 / 1440 = 0.02083333.
Final code:
$.cookie("example", "foo", { expires: 30 / 1440, path: '/' });
I've added path: '/' so that you don't forget that the cookie is set on the current path. If you're on /my-directory/ the cookie is only set for this very directory.