Change the default 10-day expiration for the password protected pages cookie

You should be using add_filter( ... ), not apply_filters( ... ):

/**
 * Filters the life span of the post password cookie.
 *
 * By default, the cookie expires 10 days from creation. To turn this
 * into a session cookie, return 0.
 *
 * @since 3.7.0
 *
 * @param int $expires The expiry time, as passed to setcookie().
 */
add_filter( 'post_password_expires', 'wpse_custom_post_password_expires' );
function wpse_custom_post_password_expires( $expires ) {
    return time() + 30; // Expire in 30 seconds
}