When can you apply for your first extension of stay on a Non-immigrant B visa in Thailand?

You can apply for your first extension of stay on a Non-immigrant B visa when your current stamp is approaching expiration. Most immigration offices allow applications for a 1-year extension when you have 30 days or less remaining on your current stamp. However, some offices, like those in Bangkok, Chiang Mai, and Phuket, permit applications when you have as much as 45 days left.