How often do you guys calibrate/replace PH probes?

[lewk]

Just curious how often everyone calibrates their PH probes. I noticed recently that mine was running on the high side, over 8.4 for most of the day, so I ordered some calibration solution. After calibrating the APEX again, dropped it back in the sump and I was at 8.15. I knew something was up when it went to 8.51 when my ATO kicked on. How long do these things typically last?

Thanks in advance!