How often do you or should you microadjust

A friend of mine has a 5d2 and a collection of Canon L primes and Zeiss primes. He is meticulous about his microadjustments and said that he's noticed changes requiring readjustments every so often. For instance, his Zeiss 50 1.4 just changed from a -2 to a +7, which he noticed after a recent trip when the focus was off with the 50 1.4, despite having microadjusted it several months ago.

I'm wondering if this has been studied at all, as far as focus changes that require readjustments, and if people typically redo the microadjustments every so often. I know I don't...unless I happen to notice that a bunch of shots with a particular lens look off.

I do it/did it once and noticed it needed an adjustment a few months later. I always double check before a big job or if I notice something is off or looks obviously soft during the shoot. I've only needed to do this with the 35L though as my 135L was spot on and the Sigma 85 (after the backfocus issues was fixed) was set at -2 for 6 months with no issues.

I did my 300 f2.8 once when I first got it. Again when I realised that a 45º target was a stupid idea a few days later. And again when I bought a new TC about a year later. No change in the correct value.

Answer: TWICE

Once to screw it up right out of the box with a calibration tool, . . and then Second, to reset to ZERO [*rim shot*]

Just kidding (Kinda), . . but I will add that lately I have been more satisfied dialing in (seldom needed any with Canon glass IME) in a variety of real world situations of the lenses intended use testing. Especially with my longer prime/TC combinations - wash, rinse, repeat as needed ~ My SpyderLENSCAL (excellent tool actually) sits lonely in a drawer

Calibrate annually.

I gues once a year or so. Or if you bang it hard on something.