How long does it take to process a Non-B visa in Thailand?

Are you already applying? If used the agent will be one day get the passport back and got 14 days pending visa for results ( should not fly out you need to send the passport back to immigration again to get the results 3 months visa) after get 3 months stamp you can fly out but don’t forget to do the re-entry permit otherwise your visa will be automatically terminated, don’t fly more than 2-3 months otherwise you will don’t have time to apply work permit, if you don’t have work permit you cannot extend 12 months business visa

You normally need to leave the country to apply for the non Imm B. If you meet the criteria and can apply in country it's usually issued the same day you apply.

Only your employer can tell you how long the documentation will take. When you get it you can probably go to Singapore to get your Non-B.