AFAIK there is no scheduled replacement interval. extanker is spot on then with his reply.
With a suitable OBD2 code reader/scan tool you can trigger some built in (built into the car's DME) O2 sensor tests. The engine/sensors/exhaust should be up to operating temperature.
If the tests pass then the sensors are probably good to go.
Or at 80K miles you can just replace them. Replace all 4. They have 80K miles and their performance will have deteriorated some. They can have another 80K miles of life left or they can, one can, die tomorrow.
I've gotten anywhere from around 100K mlles to 132K miles or more out of O2 sensors in my 2002 Boxster and my 2003 Turbo. In call cases I have replaced all the sensors when one triggered an error.