How many days before a visa extension is due can it be applied for in Thailand?

In Thailand, you can apply for a 30-day visa extension when you have 30 days or less remaining on your current visa stamp. Some immigration offices, such as those in Bangkok, Chiang Mai, Buriram, and Phuket, allow applications when there are up to 45 days left, while others may require you to have 15 days or less. Since you arrived visa-exempt with a 30-day stamp on June 27, you can apply for the extension at the Chiang Mai immigration office anytime before your visa expires, as the extension will be added to your current expiration date.