arrive in Thailand without a visa will be granted a 30 day stay if they have proof of onward ticket and adequate funds.

For longer stays you should apply for a visa before leaving home. A tourist visa allows the visitor to stay for 60 days, and a non-immigrant visa for 90 days. You should enter Thailand within 90 days of the visa being issued otherwise it will be invalid. Some countries have special agreements with Thailand regarding the issue of visas. Before travelling to Thailand check with the Thai embassy in your country or ask your travel agent for advice.

Tourist and non-immigrant visas may be extended after you have arrived in Thailand but this is strictly at the discretion of the immigration office and is not guaranteed. The 30 day visa cannot be extended, but this rule may be relaxed in the near future.

Longer term visas are available but the requirements are more complex.

If you plan to leave Thailand for a short trip, and then return before your visa expires, a re-entry permit can be obtained from the immigration office.

If you are planning a trip to Burma, Vietnam, Cambodia, or Laos, there are many travel agents in Bangkok who will arrange the necessary visa. Or you can apply yourself at the embassy.

 30 Days Visa Notes
 

Foreigners from countries qualifying for visas on arrival can come in and out of the country as many times as they like with a visa on arrival, but can stay for a maximum of 90 days in any six month period. If they stay 90 days then they must leave for 90 days before they are entitled to another visa on arrival. They can, however, go and request a tourist visa from a Royal Thai Embassy or Consulate abroad and come back into the country.

Extensions above the 90-day limit may be granted in exceptional cases, such as if the tourist is suffering from an illness or involved in a lawsuit.