С сайта посольства Таиланда: TR Visa is valid for 3 months from the date of issuance. Travellers must enter Thailand within the said 3-month period. They will be allowed to stay in Thailand for a period of up to 60 days, counting from the date of entry.
Extension is possible for another 30 days, with maximum period of stay of 90 days, counting from the date of entry.