When does CEL come on again?

So basically my CEL came on yesterday and i went over to my friends house who has an OBD-II tool and we were going to check the problem. The car was, and is, running fine, so I have no clue what it could be. So as we are doing it, he starts to frantically erase the other codes left over from other cars in an effort to get to mine as quickly as possible, and in the process, erases mine cause he is a douchebag. Since we didnt get it see it, the CEL is now off, but whatever problem was there has not been found out or checked. F*%k! How long does it take for the CEL to come back on. This time im gonna be the one using the tool on my car, so there will be no mistakes. He told me 100 miles or so, which I am still pissed about. Is he right, or does it take more?

different types of codes will take different amounts of time to come back. some will come on as soon as you start the car. some will take specific driving patterns to reset. just try and do as many different types of driving as you can. some highway, some city, some fast, some slow, etc....