Everyone's reply is very informative.
As Everyone said IT'S BETTER TO REPLACE WHEN IT FAILS coz...
When O2 sensor fails, sypmtoms are visible, and Check Engine Light turns ON. Specific DTC error code is registered in ECU memory. Symptom like rough acceleration, reduced fuel efficiency, rotten-egg smell.
Normally Toyota uses Zerconia type sensors, but platinum sensors are also found in older model like 1986. Zerconia type sensor's operational life is longer, they do not fade like brakes.
A zircon sensor works on the conduction of oxygen ions through a ceramic disc. A disc of zirconia is mounted between the gas to be measured and a reference gas, usually atmospheric air. When there is a difference in oxygen concentration between the two sides of the disc, a potential difference is generated and detected by the electrodes; this voltage is proportional to the difference in oxygen concentration and therefore a measurement of oxygen concentration can be made.
When this sensors fails, output voltage ranges outside the specified range of 200mV~800mV. When ECU senses erratic output turns the CEL on.
I would also wait until it fails. You'll know when it fails. The symptoms are obvious: CEL, egg-fart smell, poor engine performance and gas mileage.
Links are very informative.... Thanx!!!