How many miles usually before they're really hindering flow?

 

In theory, as long as the car, IF its kept in proper tune and not beat on constantly. They're designed to burn off any **** that accumulates as far as fuel deposits and stuff like that. I've seen a few cars with over 100k on the clock, and the cats function OK.

 

Well if kept in proper A/F ratios tops would be 160k usually the o2 sensor goes out and takes the cat with it.

However some cars have problems with them at 50k and usually a recall is put out on them. I have had two trucks that have come to the shop with emissions recall blocks on them and I can't smog them until they get checked out by the dealer and issued a certification of compliance.

So call the dealer if the car is within ten years old. You might get it done for free.