How long does it take to switch from a tourist visa to a work visa in Thailand?

Your question is confusing, are you wanting a non b(business) visa or a work permit? Non B can be done almost anywhere easily, usually from outside the country is quicker, you then come back and get a work permit, you can get both done inside the country but this takes just over a month, hence the reason people usually fly to a neighbouring country get a non b, fly back and get a work permit

As Garrett Greenwald said, but depending on the business, you might not be able to change and will need to obtain the first Non-B outside of the country.