“How long does it take for numbing spray to work?”

You are the judge of that. When the area that is sprayed feels numb, it is working. The timing depends on the type of local anesthetic, the concentration of local anesthetic, and the porous nature of the tissue to which it is applied.

Part of the answer has to with the type of spray, and where it is applied. “Freeze spray”, usually for shots, is ethyl chloride and numbs by cooling. It is instant no matter where you apply it, but doesn’t last long. Lidocaine or cetacaine start numbing fastest if used on mucus membranes (in mouth and nose) and over a few minutes if applied over skin. They last a few minutes.

From Christopher Creighton