They are Ferraris. Estimates is that it takes between 15 and 45 minutes for a sperm to swim the 15-18 cm distance between the vagina and (the correct) uterine tube. And the chemical change that prevents more sperm to intrude is quick (and the 1 % or so double or more fertilizations are unviable). But typically the sperm has to wait 5-6 days, either before it dies or an ovulation takes place. Yuki could catch if there is a mistiming, or perhaps if there is sperm incompatibility (but if so she would tell them, I hope).

What takes time and is risky is an implantation of a fertilized egg. Between 6-10 days, depending on where it was fertilized, with an abortion rate between fertilization and a first trimester viable fetus that IIRC has been estimated to many tens of percent – sperm have it easy.