How long do you have to leave Thailand after canceling an ED visa?

Did immigration refuse to give you an extension so they cancelled it? or you just want to cancel it?

In chiang mai they gave me 21 days to leave after cancelling which was quite generous!

As a rule you go to the immigration office a week or so before the termination of study paperwork from the school to cancel the extension. The office will cancel it and stamp you in until the termination date on the paperwork <- then you have until then to leave

BUT

some offices cancel it the day you show up (no matter what the termination paperwork says) and in that case if you can't leave that day you have to buy that 7 day 'get out of thailand' in a week stamp for 1900baht

You have to leave on the date that it is cancelled, but you can cancel it a few days in advance, gives you a little time to leave or make other arrangements.