Backups are crucial to the smooth operation of any website, no matter how important, how many visitors you have, or how frequently the site content changes.
The frequency at which you should take backups does depend on those factors, as well as exactly what kind of sites and software you have on your account.
If you have any web software installed that depends on a database to function, then you should take backups of all your databases at least once a day. The databases are usually the most frequently changing part of your hosting account.
If the other files on your hosting account are likely to change often, you should also back them up every day as well. Otherwise, once a week or once a month, even once every few months should be sufficient. Just make sure you keep the backups in a safe place, preferably on some sort of disc, like a CD/DVD or USB/external drive.