How often should one service headset bearings? 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?
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).