On most of these cartridge bearings there's no option for maintenance. Consider them sealed modules that are used until they die. If you have good hand skills, it's often possible to pick off the seal without damaging it and thereby service the bearing, or in some cases you can replace with bearings sealed on one side only, allowing future service.
Otherwise use them until they die, which varies tremendously based on bearing quality, seal integrity, weather, environment, and total rigidity of the system vs. riding style. Some people get 10s of thousands of zero maintenance miles, others can't get a season.
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