How often to back up computer?

I'm a casual user: don't store thousands of photos; not a gamer; mostly Yahoo and Twitter, shopping, browsing, personal finance, email, etc. Just bought an external backup drive. I use it to keep a mirror image of my computer, and to back up my files. In your opinion, how often should I perform a backup?... show more I'm a casual user: don't store thousands of photos; not a gamer; mostly Yahoo and Twitter, shopping, browsing, personal finance, email, etc. Just bought an external backup drive. I use it to keep a mirror image of my computer, and to back up my files. In your opinion, how often should I perform a backup? I'm looking for a time interval; I know I can do a backup if I substantially change or add anything.

I routinely backup my smartphone and GPS onto my computer every 45 days.
Update: Thank you, Dunbar. Will remember "incremental."
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