1999 Mercury Cougar catalytic converter

I have a 1999 mercury cougar, try to pass the emission test but fail because of the catalytic converter. I had it replace 2 days ago, the mechanic told me to run the car for 200 miles them go back to re-test. after a 50 miles run the check engine light went on again. I went back and he said that probably need to clean the emission system with the STP fuel cleaning fluid. I did and my worry is that the light will go off again. My question is do I really need to run 200 miles before re-testing or I can do after 50 miles or so. how long that is take for the computer to collect enough data to pass me the inspection? thanks.