Added a settings field for the plugin https://wordpress.org/plugins/wp-keep-me-logged-in/

rk-keep-me-logged-in.php

<?php
/*
Plugin Name: WP Keep Me Logged In
Plugin URI: http://www.ruudkok.nl/wordpress/keep-me-logged-in/
Description: This plugin will increase the session time of your WordPress login cookie to 1 year. That way you won't have to login over and over again. I was inspired to write this plugin after reading <a href="http://wpengineer.com/2064/stop-wordpress-from-ever-logging-out/">Frank Bültge</a>'s blogpost.
Author: Ruud Kok
Author URI: http://www.ruudkok.nl/
Version: 1.1
*/


add_action( 'admin_init', 'rk_add_keep_me_logged_in_setting' );
function rk_add_keep_me_logged_in_setting() {
    add_settings_field(
        'rk_keep_me_logged_in_setting', 
        'Keep me logged in for (days)', 
        'rk_keep_me_logged_in_setting_callback', 
        'general' 
    );
    register_setting( 'general', 'rk_keep_me_logged_in_setting' );
}

function rk_keep_me_logged_in_setting_callback() {
    $value = get_option( 'rk_keep_me_logged_in_setting', 365 );
    echo '<input type="number" name="rk_keep_me_logged_in_setting" value="' . esc_attr( $value ) . '">';
}


function rk_keep_me_logged_in( $expiration ) {
    $days = get_option( 'rk_keep_me_logged_in_setting', 365 );
    return $days * DAY_IN_SECONDS;
}
add_filter( 'auth_cookie_expiration', 'rk_keep_me_logged_in' );
?>