All of my hubs are cup-and-cone and I overhaul (disassemble, clean, sometimes new balls, new grease, reassemble and adjust) them every 6000-7000 miles or about every 2 years on each bike that is ridden in mostly good weather. I overhaul the hubs on my rain/beater bike annually as the mileage is much less but the conditions are far harsher.

Hubs with cartridge bearings are usually left alone until the bearing show looseness or other signs of distress and then the bearings are just replaced.