How long do I have to leave Thailand after cancelling my EDU visa?

totally depends on when your current extension expires. I doubt you're on any VISA at all and are most likely on an extension based on education <-(read your stamp it should say 'extension of stay permitted' with a date)

You get a letter from the school stating you stop studying on xxx date <-(which can't be any later than your current extension expiration date) you go to the immigration office a few weeks before that date and they will cancel your extension and stamp you in until the date on the termination of study paperwork. THEN you can leave.

So what that means is you can't get any more time than your current extension is valid for when you cancel it.

Actually I take that back you can get 7 more days for 1900baht, and it's not an extension stamp, it's an extension application denied stamp that gives you a week to leave.