To answer that question I think you have to ask yourself… how devastating would be if I lost everything. Some websites are very simple sites (internet business cards) while others (online stores) are very complex with hundred or maybe thousands of pages and products.

I personally think the answer would be… however often that is needed to allow quality sleep!

Here’s how it works:
If your website gets hacked or even experiences a server failure (not often but it could happen) then we would use the last backup file to restore your website. If that was a month ago and you’ve added data every day… oops, you just lost that last months data.
Your website is always backed up monthly (at least at Kirtland Media) and so if you aren’t adding anything new then there is little reason to backup more often. Now a ecommerce store or community forum is different. They add new information constantly and therefore should consider a more aggressive backup plan. Call it insurance or preservation of data. I have clients that backup daily and they sleep very well.