About four years with a Bluelab portable PH pen. I’ve had no problems with those and it stays calibrated for much longer than I would’ve expected initially. If forget how much the kit cost for a probe and EC pen, somewhere around $100 USD from the local grow store.
Many of the common issues with the probes have to do with keeping them hydrated and clean while in storage. Occasionally check the calibration. Once done using a probe for the day, a quick rinse with tap water and then store the probe tip in a bit a fresh storage solution (built into the cap of the Bluelabs pens). They are precision tools. Don’t let them dry out. Don’t shake them wildly. Don’t throw them at the television. 
That said, PH with strips or drops is perfectly acceptable and will accomplish the same end-goal. In fact, some of the scientific spectrophotometers use the same technique to measure PH along with being able to measure nutrient concentrations.
I do have strips that I keep as a backup and sometimes as a verification but my goto for portable measurement are the probes. If one wants to increase automation and data collection, probes are really the only way. You can literally see when unexpected problems start to occur in real-time. Yes, you have to maintain them, they cost more, and they can be fragile. Collecting traceable data, want a certain level of precision, building for automation … probes.
There is a good discussion on the various PH probes, cost vs quality, maintenance, etc over here: