I have a 2 HDDs in my system, 1 for the OS and another for data. I keep an image of the OS disk and re-image approximately every other month. The data disk get backed up every week. I've been burned in the past by not having frequent enough data backups. However, it really depends on how much data you generate and how important it is to you.
Ever since I lost all of my business's data years ago, I backup at least weekly. I backed up last night because of the work I did on my computer. I have two external hard drives and I use those regularly. I found an external drive - 1.5 T at Wal-Mart a couple weeks ago for $60! The model was leftover from Christmas 2010 - they had two of them - I'm kicking myself that I didn't get the other one!