On board diagnostic, how long it will take to drive it and at what speed for the system to reset?

I just purchased this van and to transfer the title I need to pass the emissions test. I took it in and I was told that it reads "not ready", and it cannot be tested until my vehicle emissions systems becomes "ready".
I would need to find out how long it will take to drive it, and at what speed for the system to reset. Can you give me those instructions, and how will I know when it is ready?