If it ain't broke don't fix it.
These's only one real guideline for sharpening anything, and that is to sharpen things when they get dull.
If your edges aren't dull, or dinged, or rusty, then they likely don't need to be serviced.
Inspect your bases and edges for any nicks or gouges regularly, if you take your board or skis to the shop to get waxed they typically sharpen them at the same time. When I sharpen my skis I check the edges with the backs of my thumbnail. If the edge shaves bits of nail off–then it's sharp, if not–then it's dull.
Typically the biggest indicator that it's time to service your board or skis is when things don't feel like they're going well anymore while you ride. Or you've hit a big rock or stump and you've done damage to your bases. That's if you're a casual recreational rider. If you're a serious racer, then you're probably going to get your skis serviced before every race.