To get back to Mr Smith's original gripe it seems he is miffed because somehow
the swing seems to be a special mission as it is out of phase with the rest of
his maintenance and he has had to go out of his way to do it.
Do it on your annual or each 2nd annual inspection?
If it takes more than 10 minutes I would be surprised. 4 cardinal points is all you
need to "check" it.
If not split the difference on the cardinals, adjust accordingly, swing on the points
of the correction card, average the errors to give you the lubber error and then
write up the card. Doddle.
If the juice has leaked out replace the gasket and fill with Jet A-1 that has stood for
a while to get the bubbles out. Stoddard solvent, mineral spirits, kerosene is also OK.
Your poor cousins to the east of you allow any LAME or Commercial pilot for that matter to swing a compass.
These days in the current machine I just swing/check it off the AHRS!
The regulations say 24 months minimum but I would say that as your aircraft would have an approved schedule
and possibly IFR it could be 12 months and you are obliged to follow the most restrictive - results may vary.
And the aircraft needs to be running and away from magnetic interference hence the compass rose at most larger airports.
In case you wondered why the Jetbox/Longbox compass is on the RH doorpost ask yourself where the battery cables are and you
will realise why it is not mounted on the windscreen centre post.
Does a compass get out of whack over time? Yes depending on what it is subjected to - see above.
If parked near a large steel object, as in my case a large steel boat, then yes and it screws the fluxgate as well but not for long.