I calibrate every day of use because, why not?
We calibrate each pH meter at least once every workday, and record the calibration data in a logbook (spreadsheet).
This is a good SOP if you want any degree of certainty in your measurements. Reliable data is good data.
I’d say that in my case pragmatism took over since it is extra effort for me to get out the calibration solutions, heat them to 25C and test the pH meter. Since I do that before experiments, the data for those needs to be more reliable than the pH readings for brewing, and I never had to change the calibration by more than 0.02 I stopped calibration and even checking before using the pH meter to take mash, wort or beer pH readings. If I get a much unexpected reading I’ll for sure check the calibration.
However, I do recommend that you start out with checking and calibrating the meter more regularly until you develop confidence that it doesn’t drift that much. With the automatic calibration meters I also suggest that you test the buffer before starting the calibration process. This way you can see how much off the pH meter was before it was calibrated again.
Interesting suggestion about storage in buffer, or a mix of buffer and storage solution. I learned something. From my experience I know that I have to regularly change the storage solution since I do get some mold growth.
As a side note, I’m actually very pleased how well my new pH meter (Milwaukee SM101) has been holding up over the last 2 years that I had it. The reading stabilized fairly quick (within 5-7s when testing wort or mash) and I have not seen a degradation of the slope (repeated need for calibrating the 4.00 point). All that after having easily done more than 1000 pH readings during experiments and regular brewing