James,
I would rephrase your question, "how often do I
check my clearances"
When I have done some work on the head involving valve replacement/regrind I will check them every few hundred miles and adjust as required until they settle down. I have found that this takes a little time.
That's why I like to have a cam-cover gasket, glued only to the cam-cover, that stays in one piece
Once they are right I would do it every year as part of a big service. But you can hear when they get noisy !
I've had two head rebuilds this year, one for a pressure test for a problem which turned out to be a porous block, the second for a dropped exhaust valve in Mondello, which luckily jammed in the port and spared the (new) Dave Bean piston (in the new block !). The valve cut-outs were marginally too small diameter for the 1.4 exhaust valves, even though the engine turned freely by hand.......
Leading the Irish Hill-climb Championship Historic Class after the first four events, I ended up third at the end of the year
But everything is now go for next year !