They say once a month, but I tend to do it when I think it necessary as I tend to not use my battery very often and thus it would logically not be very different between usages... I think.
It doesn't come down to how often you think you use your battery because unless you physically take it out, you're probably using it more often than you think. Having said that, it's probably idea to calibrate monthly but doesn't matter too much if you can't be bothered doing it that often.