Thank you for asking such a fascinating question. The survival of sperms outside the man's body depends largely on the circumstances and environment that they are exposed to. On a dry surface, such as clothing or bedding, sperms are dead by the time the semen has dried up. Sperms can remain viable for up to 5 days when they are ejaculated within a woman's reproductive tract. In a nutritive liquid like a culture medium in a laboratory, the sperms can also survive for several days.
However, here the scenario that you pose concerns the mouth and saliva. Saliva is hostile to the sperm cells. It is quite hard to predict as to how long the sperm cells can survive in such an environment. There are no proper scientific studies that have been done so far.
May I suggest the following: if you are worried about your partner getting pregnant, then it will be good to seek proper medical attention. If you and your partner have problems conceiving, then the best option is to seek help from a medical expert in subfertility such as a gynecologist.
Wishing you and your partner all the best in your journey ahead!
