This is getting as controversial as " oil change intervals ".
In my experience, an O2 sensor can be almost as unpredictable as a lightbulb.
Some seem to run forever, some not.
A good O2 sensor can be observed by watching the " cross-counts ", i.e. how many times it switches from rich to lean, in a given time.
Duke 2.6 correctly pointed this out earlier.
As an O2 sensor ages, it will get " lazy ", i.e. instead of, say 10 cross-counts/second, it slows donw to, let's say 5 crosscounts.
This may not result in the illumination of the Check engine light, but can have an affect on emissions & fuel economy.
Bottom line..........it's up to you to decide.

__________________
2007 C 230 Sport.