O2 sensor replacement interval varies depending on the model and era. They come in 3 versions - 30k, 60k and 100k. Yours is probably a 60k - but the site below will say for sure. Periodically Bosch runs an advertising campaign on changing out O2 sensors periodically as a preventive maintenance item - on the grounds that fuel economy is improved. I have a tendancy to forget about it and replace it only upon an O2 fault code. But these days it wouldn't take much a fuel economy improvement to pay for a new sensor.

http://www.boschautoparts.com/Resources/VehiclePartFinder/