I had a Catalytic Converter fail on my 1996 Trac. I got it used and when I had to get it smog tested and it failed. Paid 140.00 for a cat and replaced it myself. Took about a half hour. The following day, after a long drive, I took it to get tested again and it passed with flying colors.
With that being said, I never knew that the converter was bad. The exhaust did not smell any different then any other cars do. An old Girlfriend had a 1979 Mercury Zephyr Wagon with 280,000 miles on it. Original Cat. Passed smog testing better then most new cars pass.
Alot of it depends on how well the engine runs. Too rich, bad plugs, wires, airfilter, etc. and it can get pluged up. Too lean, clogged fuel injectors, fuel filter, carbon in the combustion chamber, etc. and it can run too hot and you can burn the converter up. A well running engine that has been taken care of properly and the converter should outlive the vehicle.
Things to look out for that can be a sign of a bad converter.
Loss of power. The converter is clogged and restricting exhaust flow (happened to a friend in his Pontiac Parissan <SP?>) You vehicle might only want to drive 25 MPH and not any faster.
Failure of emission testing. For example, NOx readings too high with CO high also. The three way cat has an element called Rhodium that has burned out. (It takes CO and NOx, removes the O from the NOX and adds it to the CO and you get CO2 and N)
Noises from the exhaust such as a buzzing or rattle in the tailpipe, not the underneith. If you hear it from under the vehicle, check the heat shields. The welds sometimes break causing the shield to rattle.
If you do not have any emmission testing in your area, don't worry about it unless you have problems.
The smell alone will not prove anything.
Tom