We recommend keeping your own backups off site due to disk space reasons. You plan only has 40 gigs drive space. Our backups will run up to 10 gigs for free off site but after that the backups will stop running.
The duration of your backup will depend on how often your website is updated and how you store your data. If you are running a database driven site like wordpress, you may want to run a scheduled backup of the database. The files are not updated as much as the database so your database info can be saved more frequently.
In a nutshell, you will want to save your site as often as you update it. Some people run cron jobs
that save their backups to a different ftp server.