[support] Setting up cron job for user role updates

Here is my wish: to run a cron job a few times a day to update all Authenticated Users to Full Members once they have posted five comments/forum posts. My plan is to have all comments posted by Authenticated Users moderated but move users up to the Full Members role I will create to cut down on moderation work. How would I go about doing this? Is there any documentation on drupal.org about setting up cron jobs? There don't seem to be any modules to allow this. Userpoints exists but isn't free. A simple cron job seems to be the easiest solution all round. Has anyone else put something like this in place? Neil