How long does it take to cancel an ED visa in Thailand before leaving the country?

The fact that you don't cancel your ED extension isn't going to impact you at a thai consulate getting a visa because the consulates aren't run by the immigration dept.

IF you get out of the country (some people are denied entry at land borders without canceling the extension) you'll get a visa fine at a thai consulate BUT you can run into real problems when you go to the immigration office for a subsequent extension on the new visa when they see the old extension wasn't canceled. I have seen people on totally new visas be charged for overstay on the old extension because it wasn't canceled.

The procedure is you are required to go cancel a current extension when you are no longer going to stay in thailand on it. That's how it's spelled out in the rules, now whether people do or don't follow the rules is on them.

Very easy in Chiang Mai, takes about 10 minutes, no lining up, as long as you have a letter from your school

The procedure when on an extension of stay based on education is to take paperwork from the school to the Immigration office and cancel your extension.

You get a paper from the school stating you are stopping studying on xx date. Immigrations will cancel your current extension and give you a stamp good until the date you're stopping studying.

See if the school will write the paper saying you're stopping on the day you leave the country.

I never cancelled my ED visa, I simply returned on another visa 2 weeks later no problem 🙂

If you're not coming back anytime soon just leave....