How far in advance can I pay the 1900 baht to extend my stay in Thailand by 30 days?

You can typically pay the 1900 baht fee for extending your Thai stay the day you go to the immigration office. However, different immigration offices may have varying rules: some offices, like in Bangkok, may allow payments up to 45 days in advance, while others, such as in Jomtien, only permit payments within 7 days. In general, many offices allow a 30-day advance fee payment.