For an article. Source: https://wordpress.stackexchange.com/questions/515/whats-the-easiest-way-to-stop-wp-from-ever-logging-me-out

functions.php

add_filter( 'auth_cookie_expiration', 'keep_me_logged_in_for_1_year' );

function keep_me_logged_in_for_1_year( $expirein ) {
    return YEAR_IN_SECONDS; // 1 year in seconds
}