At what point does swelling after a bee sting become dangerous and need attention?

If the sting is not gone or almost gone in a week, or if the sting increases in diameter over 2" or 3", get medical help. Note that this answer addresses ONLY the question of swelling. I assume we've already ruled out an emergent allergic/anaphylactic reaction and that some time has passed. It is believed that the venom produced in a bee is sometimes beneficial to human being (cases as fever, head ache...). It may be painful but not harmful!