what would make you have to replace the catalystic converter
what causes the cadalystic converter to go on 1996 Lincoln Town Car
The regular lifetime of a catalytic converter is about 100,000 miles and it will be greatly effected by the fuel quality, engine performance ( poorly maintained engine put more stress on the converter), oil quality and also the mechanical condition of the engine.
To suppliment Zee's thought -- A huge killer of converters is when your engine is wasting fuel and dumping it out the exhaust. Fuel raises already very high temperatures to a new level, and the abnormal heat causes damage.