Compass cards
 

How often do compass cards get used in service and would I be correct in saying that they only refer to the a/c stby compass?

Compass deviation cards should be available for all of the compass systems on an aircraft, having been compiled from the compass swing data. However, a decent, remote reading compass should have deviatios of no more than plus or minus 1° and so applying deviation to planned headings is a bit irrelevant.

The puddle-jumpers (PA34, PA44 etc) I fly have cards for both compasses.

Almost correct Stan,

Newer generation a/c with IRS's require a deviation card for only the stby compass as the IRS's have a 'map' of terrestrial variation in NVM which is used for magnetic heading and do not use flux valves and are therefore not subject any magnetic effects caused by the airframe.

I seem to recall figures of 3 or 5 years depending on the AMS for repeat compass swings.