How often should one service headset bearings?

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).

Until recently I would have agreed with many of the previous contributors that Headset bearings are very low maintenance. However, I have encountered a couple of instances recently where sealed bearing were seriously compromised by corrosion due to water ingress... The first was on my (less than 6 month old) Mavic UST rear wheel/hub and the other on the bottom bearing in a Condor Headset that seems to have seized completely in a matter of days after being ridden with no evidence of stiffness or binding.

Well, that is a tricky question as it depends on the conditions you have ridden in and how far... although I think conditions may be more important. I have decided to aim for around three monthly service interval since this is also when I replace tyre sealant ?

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?

Depends on the type of the headset.

If the headset is of the type that the balls roll directly against the headset cups, it is likely to produce an "indexed steering" failure -- an annoying but not-so-dangerous failure mode that makes the steering stick to some angles. It is not dangerous because a moderate amount of steering force overcomes the sticking.

If this "indexed steering" failure occurs, the proper service is to upgrade to a headset that has cartridge bearings that have a conical or spherical interface. In this type of headsets, the bearing is a unit that is supported by the conical or spherical interface that takes motions that would otherwise damage the bearings. Unfortunately, this "service" requires removing the headset cups so you need specialist tools you might need only once in your lifetime. Best bet is to visit a bike shop that already has those tools.

Most headsets sold today are of the cartridge bearing type and have a conical support for the bearings.

If you already have the newer type of headset, the only service it absolutely needs is to put in new bearings when the headset develops any kind of issue. It is probably not worth it to attempt to repair a cartridge bearing as they are so cheap anyway.

If you ride a lot in the rain and are a service perfectionist, of course it won't hurt to annually take away the fork and ensure the conical interface between the cartridge bearing and the headset cup has ample grease. The interface that prevents the "indexed steering" failure mode works only as long as it remains greased.