I work in IT. I give advice on a daily basis. A lot of it about how to not lose data. I have 4 computers here with me. I have been thinking, a lot before and now after my trip away, that I should pull new images of all of them. One never knows what might happen down here. Things get stolen. Things break. I thought about it. But, that's all I did. I have been scanning documents lately in a vain attempt to go 'paperless'. I think (hope) that most of what I scanned I also copied to an external drive attached to the computer that I used to scan. I left that computer on for a few days last week. I also use it to play games on sometimes. It's a laptop, which isn't really made to be left on, but it went into hibernation so I wasn't worried. I finally turned it off a couple of days ago. Today I turned it on to do some more scanning. It wouldn't boot. sigh.
I tried several ways to get into the drive to copy off the files. A quick check showed that my latest image was a little over 3 months old. Not too bad. Luckily my end of year guilt is stronger than the 'I'm going on a trip' guilt. I had made that backup. So, the computer is now reset to 3 months ago. Tomorrow when I'm more fresh I'll update the virus software and reinstall anything that is missing. And, I'll rescan anything that didn't make it to the external drive. And I'll image the rest of the computers and copy the contents of the external drive to a second drive. I have the equipment, I have the software, I have the knowledge. I just need to do it.
The question of how often should a computer be backed up is answered with 'how much data do you not care if you lose?'