You only replace them when they go bad. I've got a car with 300K on it and still the original cat. Length of time the job takes: under an hour. Your driving habits impacts how long a cat will last, as does how well you maintain your car.
Ken's right, I've seen catalysts outlast the car power train, with proper maintenance they should last as long as the car does.
I agree. Most converter failures are from engine problems. Such as burning oil, or an internal coolant leak. On your '05 Trailblazer if the converter is bad the check engine light will come on and you'll get a trouble code P0420 (catalyst below minimum efficiency threshold). Even then, it can sometimes be a false alarm. Sometimes if you clear the computer with a scan tool the code won't come back. If it does, then start shopping for a new converter. HTH. -Jim
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