How Long For P0420

My check engine light came on yesterday, I took it to my work to check it and it was a p0420. I reset it unaware that ford will cover the catalytic converter up to 80,000 miles. The car is a 2005 Ford Mustang GT s197 4.6 liter with 79,895 miles. Because I reset it, will they still be able to find it and if not how long will it take for the code to come back on. The technician said that at 80,000.01 miles they will not warranty it. I did however take a picture of the code and the date as well as the mileage just in case. So how many cycles or mileage will the code turn back on.
Thanks Everyone!!

Went to ford this morning and they told me that there was no codes evident on my car. I showed them the picture of the code I pulled with an OBD II reader and they said unless they can get it to display there is nothing to do. I said so if I drive back this weekend to go home, which is a 200 mile round trip and I will exceed 80,000 miles, and the light comes back on there is nothing you can do. They said that is correct. The only thing they found was a pending misfire that may be approaching and that I should replace the Coil and Plug. But that they found nothing wrong with the exhaust system. I am just upset that since I came to them with a problem regarding the Cat before the warranty runs out, and they couldn't find anything and basically they said you better hope that the light comes back on before you go over 80,000 or ford will "wash their hands of it and having nothing to do with it". So I made sure they made a service record of the visit just as record that I was there and see what they will do. Did I mention it costs me $89.00 for them to tell me basically nothing I did not already know...