I believe AC 43.13B can give you a little guidance on that matter.



AC 43.13-1B lists several instances when a compass swing must be
performed. These include:
Whenever the accuracy of the compass is suspected

After a cockpit modification or major replacement involving ferrous
metal

Whenever a compass has been subjected to a shock; for example, after a
hard landing or turbulence

After aircraft has passed through a severe electrical storm

After a lightning strike

Whenever a change is made to the electrical system

Whenever a change of cargo is likely to affect the compass

After an aircraft has been parked on one heading for more than a year

When flux valves are replaced.



An often overlooked matter amongst us aviation technicians.

Lane