Cessna, at least, requires alternator brush inspection every 500
hours. Most alternators have to come off and be opened up for this, and
bad bearings, which are normally responsible for seized alternators,
become apparent at that time. I know there's a lot of alternators out
there not getting the service they need, because it takes an extra hour
or so and costs $15 for brushes. Bearings are cheap. Magnetos need the
same care.
We normally find that alternator brushes go 1000 hours, and mag
points about the same. Bearings should be good to TBO, but if it takes
20 years to get there they dry out and could make trouble.

Dan