[ninjaofawesome]

One full day a week is a great start, we had "refactor fridays" at one of my previous jobs.

I'd say for me, its when I can see that I can make something more efficient or spot obvious mistakes or slipshod code. So it could be once a week, once a day, once an hour.. or months later when I come back to something and see that I could approach it differently.

Regardless, I like where your thought process is going on this, that you're always looking to make your code a bit better whenever you can. :)