Depending on the type of site that you run, you will need to choose a backup option that fits your needs. If you run a forum, news site, or any other site with current information you will want a daily backup, at the very least.
However, if you operate a simple personal page, that has not changed in over a year, keeping a backup of your most recent changes will suffice. A good rule of thumb for the casual webmaster is to do a complete backup every week or so.