How much time can I stay in Thailand after cancelling my ED visa?

obviously you can't get any more time than you're stamped in for

Most people get the letter from the school stating they stop studying on xxx date (usually the last day of their extension) and they go to the immigration office a few weeks before that date to cancel the extension and immigrations will stamp you in until the date on the letter.

SOME offices cancel it the day you show up no matter what the letter from the school says and they make you BUY a 7 day extension denied stamp for 1900baht that gives you a week to get out of the country