WordPress (twentyfourteen) logs me out whenever I close the browser

I’ve been tinkering with cookie settings to try to extend my login time (I’m trying to make it 6 months), and nothing has worked. I tried a plugin (Configure Login Timeout), I tried adding this code to functions.php in my theme directory:

add_filter( 'auth_cookie_expiration', 'keep_me_logged_in_for_10_years', 1999);

function keep_me_logged_in_for_10_years($junk = 0) {
    return 31556926 * 10; // 10 years in seconds
}

And I tried adding it to functions.php in the twentyfourteen theme directory too. Session length is 6 months in my php.ini, and I know it works because it works on my other sites.

It’s driving me nuts. Could someone please tell me the exact line and file that controls login length so I can go at it with a hammer?