Thailand Visa Validity / Duration of Stay:

TOURIST VISA: If you plan to stay more than 60 days you must apply for an extension of your visa after you arrive in Thailand. To do so, you must obtain a Tourist visa in advance through Thailand Embassy/Consulate in the USA.
BUSINESS VISA: With issued visa, you can stay for up to 90 days.

Visa on arrival through border crossing:
If you enter Thailand from a neighboring country by crossing a border, a 15-day stay visa is issued rather than a 30 day stay visa.