Can be a couple of days. Depends on how your immune system decides to react. Some are stung, get the welt swelling and pain, then it subsides within an hour or two. Some are more sensitive to the actual poison in the bee sting and they will be affected for longer. At times a person can have a mild allergic reaction and the symptoms last longer because the body is having a more difficult time dealing with it. It is not really a concern unless it persists for much longer. Keep icing it to reduce the swelling and take some Ibuprofen.