How early can I apply to extend my 60-day tourist visa in Chiang Mai before it expires?

In Chiang Mai, you can apply to extend your 60-day tourist visa during the last 45 days of your stay. This means you can visit the immigration office any working day between now and your visa expiry on 28 March. It's recommended not to wait until the last moment to secure your extension, as early applications do not affect the validity of your stay.