How long will wheel bearings last? Depends. If you hang your bike on a rack and drive in the rain at highway speeds, then every bearing on your bike will be shot in short notice.

OTOH, I rode a set of Campagnolo Tipo (cheapo) hubs for over 25 years, during which I wore through 4 successive sets of rims. Unsealed, but grease injection ports allowed me to purge the hubs completely after every wet ride. In a weak moment, I sold these; I assume the hubs are still going.

I'm looking at a set of new Vision TC24 carbon tubulars that I pulled off of a rack. The hubs seem relatively well constructed and sealed. The freehub design is not as good as Shimano's, so possibly water can get into the gap between the freehub and the hub shell. And the hub design depends on cartridge bearings, again, not as good as the Shimano and Campagnolo implementation.

Maybe some day I'll mount tires on the Visions - at 1,200 grams or so they're reasonably light. The rims are wider than my other tubular wheels - a PITA that will require me to adjust my brakes. ...The reason why they've been sitting on a rack for 3 years.