Even cartridge bearings need maintainance, not as often but they still need service. I just did some work on my friend's bike, I checked his Chris King headset and hubs, the cartridge bearings were dry. His assumption was use them till they need to be pulled and replaced. If you open up sealed cartridge bearings once a year look them over, clean if necessary and add grease they will last a long time.

I have found that some manufacturers under lube their new components. I installed 3 new Shimano headsets on project and friends bikes this year, I had to add grease to each of the sealed bearing cartridges. That just shouldn't be the case. When you turn cartridge bearings you should feel a smooth drag, that tells you they are well greased and that the seals are tight.

The nice thing about cartridge bearings is that when they do go bad you can replace everything in one step.