Replacing/repacking any of the bearing systems (headset, BB, hubs) should be either once a year or as-needed depending on mileage and riding conditions. If your bearing system has no play and is rolling smoothly, then it likely does not need any service. If it feels "crunchy," has some noticeable resistance to movement, or has any play that lets your steerer/spindle/axle move around, then it needs to be serviced. I'm pretty sure Park Tool has some instructions on how to properly inspect all of your bearings, but that's the gist of it.