Typically the time to diagnose a defective Oxygen sensor is about 1 hour. This involves test driving the vehicle at least a little bit, reading the code, writing down the freeze frame information, visually inspecting the sensor and its connections and lab scoping the output in terms of voltage bandwidth and rise time from its leanest readings to its richest readings. Some people will cut corners and just pull a code and then recommend a sensor, but this isn't very professional. In California when you do a Diagnosis for an Emissions Test Failure, even if there are no codes for the sensor, the steps I just described are the bare minimum.
The Front Oxygen sensor takes about .3 hours and the rear about .5 hours. Be SURE and let the car COOL down or you will really, seriouly burn your self. Then clear the code and do a monitor setting test drive which is basically driving at 55-58 mph for about 4-5 miles with out interruption. This sets the Oxygen sensor monitors very fast.
What about the hours for replacing it? It looks fairly accessible to the front of the engine, just below the exhaust manifold and with a quick disconnect cable. Are there any complications? e.g. it's in a area of intense heat with the attendant breakdown of materials which could impact removal.