Why? Thailand is extremely affordable and tourist friendly. From the jaw-droppingly gorgeous beaches to cheap delicious street food, Thailand is a dream land for budget backpackers. High end options abound as well, and due to the low cost of labor and supplies in the region you can expect 5 star rooms and service that are much more affordable than many other destinations. Don’t be turned off by the recent political unrest. The Buddhist nature of the protests and the importance of tourism to the economy means that tourist are rarely affected by the conflicts and even then it’s a minor inconvenience at most.
Where? If you have the time and want to get a good sense of the diversity within the country, you should plan to visit at least one place in each of three regions: Northern mountainous area, central area around Bangkok, beaches and islands. There is tremendous variety within each of these regions, but you’re definitely missing out on part of the Thailand experience if you don’t squeeze in at least a couple of days and meals in each region.
When? The best months weather-wise are around November to March. This is the “cool, dry” season, which is still plenty warm and steamy by American or European standards. July to September should generally be avoided because of monsoon weather and the accompanying shutting down of some tourist services.
How much? Thailand is one of the least expensive countries in the world for tourists to visit and there is ample tourist infrastructure and services. Bare bones it for less than $35 USD per day. Enjoy modest air-conditioned accommodations and sit down meals for around $60 USD per day. Live like kings for around $200 in most places, more at high-end international beach resorts.
How long? The most expensive part of visiting Thailand from most English speaking countries is going to be the cost of the flight there, so the longer you can stay the more economical it actually is. My first trip to Thailand was 3 weeks, which allowed enough time to experience the island beaches, go on a hilltribe trek, visit Bangkok and Chaing Mai, and squeeze in a few days at Siem Reap and Angkor Wat over the border in Cambodia. Relatively cheap flights around the country will allow you to see it faster for a little extra money, but buses and trains are dirt cheap and allow you to see the countryside.