A couple of years ago Thailand changed the 30 on arrival visa, you could only get 3 of these visas in a 6 month period. The idea was to stop people living here in Thailand on constant visa border runs. After the flight in and 2 visas border runs to Cambodia, the tourist would not be able to reenter Thailand for 3 more months, unless they obtain a Thai tourist visa, Non O Visa.

Border run in Thailand

The latest change allows at any road crossings into Thailand 15 days on arrival, if a tourist arrives via the air then 30 days on arrival still remain.

The latest change to the visa border run in Thailand at first seems like a bad idea, or that it is trying to keep westerns away, but if you really look at it, this latest visa change is a good change and in the right direction

Firstly over the last 10 years many tourists have lived in Thailand on the Visa on arrival and then carried out a visa run via the lady crossing in Cambodia every month. Living in Thailand this way was making a mockery of the visa rules. The visa on arrival was never intended for tourists to stay long term.

The change in 2007 set out to stop the misuse of this visa and Thailands good will. After all for people with a reason to stay in Thailand could easily obtain a multi enter tourist visa or Non immigrant O visa, which would allow the holder to stay in Thailand from anywhere between 3 months and 15 months.
The latest changes to the Visa rules now allows a traveler to enter Thailand if by air for 30 days and then a 7 day extension can be given at immigration. In total 30 + 7 = 37 days. After this time the tourist can either return home, visit a neighboring country or do a visa run.

15 day visa from Cambodia visa run

A visa run at a land crossing like Cambodia will now add a further 15 days Thai visa to the holders passport. But the old rule has been removed allowing the person to obtain as many 15 day visas as he requires.

Staying in Thailand

Until the visa rules are changed again, this is the amount of time a person without a Tourist on non immigrant visa can stay in Thailand
30 Days on arrival
7 Days extension at Thai immigration office
15 day visa at Cambodia border
15 days visa at Cambodia bolder (the limit to border runs has been removed so this is endless)