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. Some web software programs have a backup system built in.
If the other files on your hosting account are likely to change often, you should also back them up every day. 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 or DVD, or an external hard drive.
Even though you may think nothing is going to happen to your information, you shouldn't assume that everything will be okay. Hard drives fail more frequently than most people would think.
Almost every control panel has some sort of backup system in place. Some will let you set up automatic backups, and most will allow you to take a backup manually. If there is no sort of backup mechanism, you can always take a backup manually via FTP.
Many web hosts will take frequent backups of all the accounts on their servers, but you shouldn't count on that. The more backups you or anyone else takes of your account, the less likely you are to lose important data.