There’s not so much a service interval as an “as needed” basis. Cars are pretty well sealed up so it’s easy to specify a service interval. Bikes are not so well sealed up so you may be able to go the whole summer with no work but every other month in winter.
Take the wheel out and spin the axle by hand. Does it feel rough or notchy? If so, time for service. Also be aware that a lot of wheels now have sealed bearings so it may be a matter of replacement and not service.