I started shaving when there was enough hair growth to justify it either by saturation or length. Initially, the facial hair I had was really patchy so I would shave the patches every other week. In that time, the hair got to be maybe a centimeter long.
Now that it's more spread out on my face, I shave every Sunday night and that usually lasts me for the week. I don't work in a place that requires me to be clean shaven. I do my sideburns when I shave my head once a month or so.