Oxygen sensors

I own a 98 4-door LS blazer with the standard 4.3. I currently have 40K on the odometer. What is the normal mileage interval before replacing the oxygen sensor(s). I believe there are two, one near the exhaust manifold and one near the catalytic converter. I recently aquired the vehicle from my deceased uncle's estate. Please advise.

Thanks,
C. Kolbosky

There really isnt any specific time at which you need to change it, basically when it goes out. So, if you get a code then you need to replace it, other than that you should be ok.