It depends on how much spit you exchange with your girl/guy. It can stay for couple of minutes (if you brush your teeth) or couple of hours without any mouth cleaning.
How Long Do Spit Stay In Your Mouth From Kissing?
I don't know. That's uh gross.
I disagree I heard that it can stay in your mouth for a whole year and you can check to see how many people have kissed you over the 1 year....