I think this is dependent on the pre-TAR inspection looking at
+ steam trends
+ HPU pressure/flow/history
+ boriscope
+ history of operation
The longest I have heard is an overhaul every other TAR but most people do a overhaul each TAR.
Wear sites and locations varies between the TUD and BUD and the various generations of design. Seats, seals, and actuator components can see wear. The gates and body rarely see wear but it has happened for various reason. So it really comes down to your pre-TAR inspection.