Personally I think this depends largely on what types of community you're going to be around the majority of the time. If you're talking about living day to day, ordering food and getting around, you won't need much Thai to actually live here. Ordering food will require some pointing and sign languages at street food stores (most of them understand English numbers + sign language), but at restaurants in malls, English is "well-enough" understood and you won't have any problems (or they'll call the manager to talk to you.) Now of course, if you're in the startup scene, you're probably going to know some locals that can speak both languages well and are willing to help you around, however, you should alway try to learn the local language enough to get by on a daily basis.

In the startup community and the white collar working population, we understand English well enough to understand what you're saying but maybe not converse with you fluently 100% of the time.

EDIT: I'm only referring to Bangkok in this case.