Originally Posted by Harley Condra

If you intend to fly beyond the recommended 25 hour service interval, upon each shutdown, keep track of the "spool down time". i.e., the amount of time it takes for the rotor to come to a complete stop after shutdown.
If the spool down time becomes shorter, for example, if it drops from 15 seconds to 10 seconds, it is an indication that the bearings have worn, signaling that service is required.
Another sign of bearing wear is noise...bad bearings sound noisy when shut down.