Aroobie,
For what it takes to unbolt the calipers, I would at least look at them at every oil change. And, since I got only about 4800 out of the stock rear pads, I make it a point to check them on the bike every 1000-2000 miles or so.
~broken record mode~
A lot depends on where you ride, how you ride, etc....
I found a small rock wedged in my front caliper when I checked it at my 8000 mile oil change, but they had just tarred and chipped a few local roads prior to that.
Do what makes you comfy...
