I calibrate every 6 months. The pH probes tend to drift a bit over time and calibrating them on a set schedule helps alleviate the drift. If you are ever in doubt of whether you need to calibrate or not, just buy the packets of pH calibration solution and stick your probe in it. If it is the 7.0 packet and your probe doesn't read 7.0, you need to calibrate. If you're dipping in the 10.0 and it doesn't read 10.0, you need to calibrate. I've never heard of not calibrating your pH probe, but then again, I used to do environmental water testing for the EPA so they have stricter standards.
The calibration packets are about a $1/each and it takes 5 mins to calibrate. I honestly don't see why you would not calibrate to keep your readings on point.
To the OP first question, I would keep using the same probe until it can't maintain calibration anymore. We averaged a good 2-3 years from our probes used in the groundwater sampling equipment before they couldn't hold calibration anymore. Currently, I've had my 2 probes in my tank for over 4 years and they still hold the calibration well so I've had no need to replace them yet. When I do, I'm probably sport for the lab grade one to use with my CaRX since precision is very important in that scenario.
