It's good practise not to hardcode any values that are likely to change directly to your programm code. So in index.php VISITORS_DURATION was defined as constant, so it could easily be spotted and changed. To make changing this setting even more convenient for users, you can put it to a configuration file.
So create a folder config/ in your plugin's folder, and create a file config.php with the following contents and save it as UTF-8 without BOM.
<?php $plugin_cf['visitors_online']['duration']="5"; ?>
This is very similar to the language file. Note that the configuration variable is called $plugin_cf instead of $plugin_tx.
To use the config variable change line 7 of index.php to
define('VISITORS_DURATION', $plugin_cf['visitors_online']['duration']*60);
TODO: help texts