The frequency to when you backup your website files depends on the type of website you have and how often the website is updated with new content.
If it's a wordpress or other blog you should backup the database every time you publish a new blog post.
Or if it's a forum or other type of website that is updated daily and constantly from users then it should be backed up at least once or twice a day depending on how much content is being added.
It's never hurting anything to backup your database too often.