When does my 30-day visa exemption extension start if I apply tomorrow in Chiang Mai?

You can apply for your 30-day visa exemption extension as soon as you're eligible, which typically means you can go to the immigration office anytime before your current exemption expires. If you apply tomorrow, your extension will start from the day you apply, which would mean it would start from April 24, following the expiration of your original 30-day visa exemption.