From what one sting therapist said at one of our beek meetings, the reaction/swelling progressively worsens after each sting. And continues until immunity is reached at which time the reactions stop.
Very shortly after getting into beekeeping, I opened a hive one day and zigged when I should have zagged and got a couple a dozen stings. So I got all my shots at once.
I can't begin to even think about how many times I've been stung? And probably a fraction of what some have had. But I have never had much of a reaction to any thankfully! But still, some sites are worse than others for swelling. Under the arm is one of those. Around tendons is another and usually the most painful, both in terms of immediate and sustained from swelling in a joint for instance.
Most stings for me never tend to last typically no more than 2 or 3 minutes, less time than it takes to find anything to put on them. I do know that the black bees seem to carry more of a punch in their venom than the Italians do. But the duration and reaction doesn't seem to be any longer.
I've never used DMSO for bee stings or swelling more precisely. I know it helps swelling and I've been using it for a while for joint pain "the wonder cure!" I know how well it works for pain, but have no idea about reduction in swelling. It is supposed to be diluted to 70 to 90 percent with water before application to be most effective. It can also be purchased on the web and is reasonably cheap to buy.
Todes is right, it is a solvent and can be contaminated easily. The application area should also be cleaned before it's applied as well to remove any contaminates that normally wouldn't penetrate the skin. DMSO and whatever it's carrying will penetrate skin and tissue very rapidly. And one reason it's popular with folks that use herbs and EO's for various reasons.
"Success is not final, failure is not fatal: it is the courage to continue that counts." Winston Churchill