Bosch recommends the following:
unheated O2 sensors 1976- early 90s every 30k-50k miles
Heated O2 sensors (first generation) mid 80s- mid 90s every 60k
Heated O2 sensors (second generation) mid 90s and up every 100k
This assumes that the car is in good running condition.
So worse case scenario you just buy a new catalytic converter. I was able to get on installed for about $220. Passed smog with no problems after that, but the nightmare I had before getting that new cat was unbelievable. I guess the m103s may be known for failing smog as mine failed 3 times consecutively and every year that I had to get it smogged it failed. So my advice is that if you still fail....just bite the bullet and get a new cat if everything else is in good condition otherwise you may be in for a battle. Also, make sure you are running the right plugs, non-resistor type and the brand specified, although I have not haad issues running the resistors, but there seems to be a divide about using those amongst many forum members. Good luck.

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1990 190E 3.0L