As soon as you have an abode rented and are willing to stay or have stayed longer then 180 days you are considered to be liable to income tax in most jurisdictions. 180 days is the definition of the thai tax law, so fairly straight forward.
I agree most people do not need a tin or the certificate but why do you point that out? I never stated I was in the majority. However more and more banks tend to demand a that tax id or at least they try hard and 50% cut you off as a customer right away.