Apparently there are some sealed bearing cartridges that can be opened and repacked with grease. But most sealed bearing systems are throw-away cartridges. If the bearings are still good, you do nothing. If they are bad, you replace them. Having the bearings checked once a year would be reasonable. Or take it in any time you notice that the bearing has play or feels rough or is binding. Most sealed bearing systems should last several years (ie. 4-5 years or more).