How long before you arrive in Thailand ahead of time, before you start work?
Will non-b visa applications generally be approved 2 weeks before date of work commences on the application, so you can travel to Thailand earlier than work?
TLDR : Answer Summary
You can apply for a non-B visa to work in Thailand approximately two weeks before your planned start date. Once the visa is issued, you must enter Thailand within 90 days, which allows some flexibility for timely travel before starting work.