Reset the gyro every 15 minutes!
That's an A-ffirmative! At least that's what I was taught over my years of flight training, check rides, and reviews.
As far as any legal requirements, I don't know (and haven't bothered to check) what's in the FARs; I'm certain there are manufacturer's tolerances for their gyro instruments....their testing/inspection info would be maintained in the aircraft logbook.
The directional gyros in the single and twin-engine aircraft I've flown did require frequent checking (every 15 min. recommended); they could drift excessively over time. When resetting the DG to the magnetic compass, you should be in wings-level, unaccelerated flight.
