O2 sensor question

Just courious, some of the auto parts sale fliers claim you should change your o2 sensor(s) once a year.If a brand new sensor would be say a 100, and a shot one is a zero, approx. where in that span would, "a" milage etc. start dropping and "b" the check engine light come on? Not that I plan on doin' it yet But with my 2 costing about $120.oo or so, just wonderin'... figgered it was "sales B.S."

The recommended interval is 100,000 miles.

The reaction time of an O2 sensor starts to slow down as soon as it's installed and the engine is operating. Once the reaction time exceeds the preset parameters programmed into the computer, the "service engine soon" light will illuminate. Typically this is around 100,000 miles. You will find "replacement recommendations" ranging all over the board. The fact is, the onboard computer will tell you when it needs to be replaced. If it's replaced before that, a noticeable difference in performance is doubtful.