We put this question to some Thailand teachers on social media. As one teacher was quick to point out - legally, you can't work a single second of time without the correct visa or a work permit. However, as another teacher said - I would allow a maximum of three months. You're allowed to work on your single entry non-B visa, as long as the company can prove your work permit is in process. Another teacher kind of agreed by saying - legally you can't work without a work permit, but we all know that's unrealistic as many schools like to stall the process to make sure you've got what it takes to become one of their teachers. I'd also plan for three months.