Sorry for posting that question, I think @wigglyamp gave the answer in an older thread, namely 24month
AMC1 to CS-ACNS.A.GEN.010 requires a check of altitude correspondence and correct Mode S address transmission every 24 months for Mode S installations and this is in line with previous national requirements for Mode C transponders. We had a UK mandatory requirement which later became an EASA AD (AD 2006-0265), later replaced with SIB 2011-15R2 (http://ad.easa.europa.eu/ad/2011-15R2) which now recommends the 24 month testing.
Although it refers only to transponders and not to the pitot – static test …
On G, IME, this test is rarely done on altimeters because not many firms have the big box for doing it.
Competent companies always do this check – it’s only the one-man-in-a-van maintenance guys who don’t have the required test equipment.
