Set the time that the cookie in WordPress will be kept at 1 year.

keep_me_logged_in_for_1_year.php

<?php
/**
 * Plugin Name: Keep me Logged In for 1 Year
 * Plugin URI:  http://bueltge.de/wordpress-login-unterbinden/1220/
 * Description: Set the time that the cookie will be kept at 1 year.
 * Version:     0.0.1
 * Author:      Frank B&uuml;ltge
 * Author URI:  http://bueltge.de/
 */

if ( ! function_exists( 'keep_me_logged_in_for_1_year' ) ) {

	add_filter( 'auth_cookie_expiration', 'keep_me_logged_in_for_1_year' );
	
	function keep_me_logged_in_for_1_year( $expirein ) {
		
		return 31556926; // 1 year in seconds
	}
	
}