Most of the Geiger counter manufacturers say to calibrate their instruments annually or semiannually, which at $75+ a pop adds up over the years.

Is it really necessary?  

If I periodically perform a test on a known long half-life sample at the same distance (like in some kind of jig), and get the same reading as when I first did it with a calibrated instrument on every scale could I assume it is still working and “calibrated”?  

Also it seems unlikely it would lose more than +- 5% accuracy at the most, unless something went bad like capacitors.
That coupled with the Geiger counters already “fuzzy” accuracy of +-20%, I don’t see why it would be necessary.
But maybe the Geiger tube itself changes over time, and gives different readings?

Thanks for any light you can shed on the subject.

"perform a test on a known long half-life sample at the same distance (like in some kind of jig), and get the same reading as when I first did it with a calibrated instrument on every scale"

and where do you get a source that will cover every scale??

Get an old Coleman lantern mantle or a old piece or orange Fiesta ware. Check your instrument on the scales that those sources will register on and call it a day. Contact readings are ok for this simple check.

If your instrument over time responds about the same with this check there is a very good chance it will work properly on the rest of the scales.

Of course be cautious if the instrument uses two different tubes. If it has two tubes the smaller one is for the higher ranges and you will not be able to easily get any meter response with these small sources.

BTW - I was in instrument calibration for almost 30 years.

A survey meter I used to use had a self check under he cover. You'd rotate it out of shielding and have some small reading.

There are very low emitters available.

Some of the modern microprocessor-based designs are "self-calibrating" to a certain extent - i.e., they have lookup tables stored in their software to correct the readings for aging.

The annual calibration recommendation is primarily targeted at professionals who use their meters all the time. A meter that sits in storage most of the time (which is typical service among preppers) isn't subjected to nearly as much banging around, environmental extremes, etc. - and thus is less likely to drift out of calibration as often.

Functionality check is more important than calibration. It it's a low range meter, buy a slightly active source as mentioned in one of the posts above. High range is more problematic but there usually is not much to go wrong with it.
I checked my HT meter at work. We have nuclear scales. I could detect it from about 50 ft away with my meter.

Are "Geiger Counters" available to civilians?  If so, where?  I haven't seen anything like that since a left the Army a few decades ago.

Ludlums
Fluke

We calibrate the county meters every two years.