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What are the symptoms of a bad oxygen sensor?
Symptoms of a Bad Oxygen Sensor:
When you have a bad oxygen sensor, your vehicle will run less efficiently, it can sometimes have a poor idle, erratic jerking at steady throttle, hard starting problems, cause the check engine light to come on, and will cause high fuel consumption.

How do you know if your oxygen sensor is bad?
Check this list to see if it's time to replace YOUR vehicle's oxygen sensors:
Sudden decrease in fuel mileage....
Flashing check engine light or malfunction indicator lamp in the vehicle?s dash board....
Failure to pass smog....
Overall poor vehicle performance; rough idling, stalling, hesitation on acceleration, etc.

When do you guys change your upstream oxygen sensor ? Do you wait until there's a DTC ?

standard recommendation is 100,000mi I personally wouldn't push it past 75,000mi....it and the MAF sensor have some of the biggest affects on your fuel mileage.

166k miles on the original O2 and everything appears to be fine. But I guess it might be time for a change soon.

Change it/them when it/they becomes a problem. Actually first, I would try cleaning them and see if that buys you some additional time.

There is no need to change them at 75k or 100k or whatever. I think that's my lulz for the day.

Never had service bulletins that told us to change HO2S's because "when they reach a pre-determined mileage..."

yes, dont throw parts at your ride just cause - if you have 100 buck scanner, you can see their voltages and then determine if they are working within the expected range and not slow or stuck - not very expensive but clean all 3 first - o2 x 2 and maf.

poverty forces one to do unorthodox things