the Tourist SIMs are normally valid for 1-3 months and then they expire automatically unless you continue to top up/add credit. They are pre pay. So no commitment period at all. You will need to provide passport or ID to purchase a SIM in Thailand btw.
You can get a Thai SIM either at airport or at 7-11 which comes with a starter amount of data. Then if you add credit either through the app of the service provider or at 7-11, you can add a ‘local Asian countries’ (which will cover Cambodia, Vietnam etc) roaming pack through the app or via sms service codes that you’ll find on the website of the service provider.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.