O2 Sensor thoughts?

Anyone with high-mileage experience on BMWs have any thoughts on change interval for O2 sensors? I'm at about 167K miles now and still on the original sensors. No error codes but, on my E39 before this car, my first warning was when the cats were exhausted (and that car had four cats. I could never pass inspection after that but, compared to the $4000+ to replace them, I could pay a lot of tickets for no inspection sticker...)

I see that our cars have four total (before and after the cat on each side) and they're in the neighborhood of $150 each.

Thoughts?