This will do a mysql dump of the FratHouse database that has been setup with the user permissions of JohnSmith and a password of Toga and place the sql dump in the home directory of JohnSmith.
THen just set the cron job peramiters to run as frequent as you want. If you want to really get complex then you can add some time/date variables to the filename so that the filename is created with a time/date stamp in the filename so when the cron job runs again it does not overwrite the previous file.