How often should one disassemble one's headset and service the bearings, assuming it's a fairly new road bicycle with sealed cartridge bearings and is ridden every day?
I'm an almost daily all-season rider for a dozen years. Press fit headset bearings are subject to wear and corrosion. If you ride and/or wash your bike regularly you will feel a slight stiffening over time (handlebars don't move as readily when walking the bike with hand on the seat). This steering symptom gradually increases to feel like you would expect from compromised bearings. Like most things that are press fit, screwed or slipped into a bike frame if they don't get busted loose every few years they will become corroded in place resulting in fits at the bike shop. And while you're at it you may as well replace them.
Probably never -- headsets can easily last the life of a bicycle for most people without needing an overhaul.
If you ride in the wet, it is advisable to have a front fender, though (to reduce the likelihood of needing an overhaul even further).