There are different areas besides the direct location to be stung for issues. Other areas include the spine, trigger spots along the nerve meridians, and scars. Many people sting areas of pain or infection, also.
I have recommended ice prior to stings many times. If you have a vacuum sealer, you can put ice cubes in a small pouch made from the special plastic that comes with the sealer, seal (not vacuum seal though) the pouch, let the ice melt (or not) and then refreeze flat. Use and reuse.
Cut the corners of the pouch and make them rounded. Pointed corners are picky.
You ask "how long?" Very difficult to answer based on information given.