As your Oxygen sensor aproaches the end of it's life, it will probably have both voltage falloff and a slower reaction time. However, the car can not tell that something is wrong until the sensor is completely dead (incapable or producing more than .5 volts and it should trip a fault code at this time). As you get more and more voltage falloff, the car will enrichen the mixture to compensate, and believes itself to be running fine.

For the greatest sensor life expectancy, always buy Bosch. I've yet to have any non-Bosch sensor last more than 2 years. I typically change my Bosch sensors after about 5 years, just to keep the optimum fuel effeciency for my vehicle.

For more information, read the following posts:
http://www.brickboard.com/RWD/index.htm?id=721861
Cost effective 3-wire O2 sensor:
http://frwilk.com/early944/misc/oxygen.htm

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 231K