About every 9-12 months, but I voted once a year. I try and time it on the lunar calendar; I don't know whether there's anything in it, but I figured it can't hurt. I normally end up trimming 3 times every two years.

I've been growing out layers for my last few trims, so it's been 2" a pop, but I'm planning to go down to 1" (or as necessary) when my layers are completely gone.