As far as I know, the longest tourist visa anyone get obtain for Thailand is 60 days. A 30 day trip to Thailand does not require any visa at all. Most people are under the impression that to stay longer you than 60 days you must exit the country and then reenter which people tend to do on the Laos or Cambodia borders by ground or to Vietnam by air and travelers build their itineraries and adventures around this. Reentering Thailand will result in a visa extension but you may not be able to control the time stamp they put in your passport, ie you might only get a week or 2 weeks. The best way to assure yourself of another 30 day extension is to go to the Immigration Division office in Bangkok.
Take the BTS Skytrain to the last stop called Mo Chit on the Sukhumvit Line and exit at Mo Chit as if you were going to the JaktuJak Weekend Market. From here, take a taxi to Government Center, Chaengwattana Road, Soi 7 for about 100 Baht. Have the taxi take you directly to Building B and then go to the 2nd floor and look for the Immigration Office.
These are the official Thai Government buildings and they are beyond huge. All Thai government departments appear to be centrally located in Building B which has an atrium inside about the size of 10 football fields. I have to say, I had no idea that I was going to see such amazing new architecture. Once inside the Immigration offices, whole process took 3 hours, so almost 5 hour trip including your transportation to and from. Yes, its busy, yes there are hundreds of foreigners doing the exact same thing but its a good place to exchange travel stories with people.
Note, if you search for Thai visa renewals online and if you find other Thai Government office addresses in Sathorn or other Thai Government office locations around Bangkok, this information is incorrect and out of date. If you go to any of these other Thai Government offices, they will redirect to the address I have given you above. This visa renewal will cost you $59 or 1,900 Baht per person. Hey, you’ve got to pay the piper no matter what country your in.