I'm not quite sure what you're asking, and I'm trying to be delicate here. You either swallow it or you spit it out, as with anything in your mouth. It doesn't continue to live and reproduce like bacteria, if that's what you're asking.
Having semen or sperm in your mouth is not a pregnancy risk to yourself, if that is why you want to know. You can't get pregnant from what is in your mouth, and as RockRose says, it's gone pretty fast in any case.
For pregnancy, sperm has to make it to the vagina. I guess in theory if someone were to take sperm out of their mouth and immediately put it into someone's vagina, there might be a risk of pregnancy any sperm had stayed protected from the acidity of the mouth, and then protected from the air, by the semen around it. But the sperm in the semen would not have long before dying due to being exposed to saliva or air. I only mention it in case you are concerned that you exposed someone else to sperm that was in your mouth.
If you are worried about getting an STD, that is another and more valid issue. Get checked after the appropriate exposure window has passed.
If it just bothers you that it was there -- sweetheart, don't worry, it's gone. Gargle and brush your teeth.
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