How far in advance should I apply for a visa to visit Thailand for 2-3 months in December?

I just got the DTV soft power April 1st! How long you should apply depends on the embassy you’re applying from, some are more responsive than others. A month in advance is pretty good in general.

I applied online at the Washington DC Embassy. It only took 5 days. They sent me a notice after the 3rd day asking for more information. I sent it directly to them. 1 day later they sent me a 2nd email asking for more information. I sent it directly to them ( I began to worry) and on the 5th day they sent my visa via email.

unless you plan to live in Thailand long term, the 5-year DTV is not the correct visa for the purpose of a 2-3 months holiday. Brandon gave you the correct reply. As a U.S. Citizen, you can enter Thailand visa-exempt (without a visa) and will get stamped in for a 60-days stay permit. You can extend these 60 days with additional 30 days on Immigration for 1900.- THB. . . . . . . . . ATTENTION: if you travel without a visa, the airline can ask you for a proof of onward travel out of Thailand within this 60-days period you will get stamped in upon entry. So you would need a cheap one-way ticket with a budget airline, like Air Asia from Bangkok to Siem Reap for 36 USD, to comply with this requirememt, should the airline ask you. Alternatively you could buy a so called "onwardticket"prior to departure from your home country airport

recommended to submit approximately 1 month in advance.

Most embassies will not process a visa application if your travel date is farther away than 3 months from when you apply. So wait until you are within 3 months of your trip.

But you don't need a DTV for 3 months. Just use the 60 day visa exempt if it's still 60 days then, or apply for the tourist visa online if they switch visa exempt back to 30 days. Then you can get a 30 day extension in Thailand, which will give you 90 days total.