I understand that, as an American Citizen (arriving in Thailand from within Asia), upon arrival in Thailand I am not required to have a visa.

Can someone tell me how long I can stay upon entry?

Also, is it possible to request a longer stay at the time of entry?

You will get 30 days visa exempt entry only by air.

A single entry tourist visa obtained before you travel would give 60 days on entry, plus option to extend for a further 30 days at Thai immigration.

30 days by air or 15 days by land and you can not request a longer stay - for that you should obtain a tourist visa prior to entry for a 60 day stay that can be extended 30 days.

Note that you will require a flight out of Thailand within 30 days if you do not have a visa issued at a Thai consulate. Without a visa or flight out you may be prevented from boarding your flight.

Where are you currently located? Some embassies and consulates are easier than others when applying for a visa as a non-national of the location country.

Note that you will require a flight out of Thailand within 30 days if you do not have a visa issued at a Thai consulate. Without a visa or flight out you may be prevented from boarding your flight.

Where are you currently located? Some embassies and consulates are easier than others when applying for a visa as a non-national of the location country.

I am in the Philippines. I would have to go to the Thai Embassy / Consulate in Cebu City, I believe, or Manila, to secure a visa.