It depends upon where the egg is. Ovulation may not have occurred yet, making it a longer time for sperm to reach the egg.
Here is an answer:
Question No. 3326 :
How long does it take for the human sperm to reach the egg? How long can sperm and egg remain viable for fertilisation?
Once sperm are deposited in the vagina, they must travel through the cervical canal, through the uterus, and up to the egg in the upper third of the oviduct. Spermatozoa can reach the distal uterine (Fallopian) tube within 15 minutes after semen is deposited in the vagina. The vast majority of spermatozoa never make it. Many are destroyed by vaginal acid or drain out of the vagina. Others fail to penetrate the mucus of the cervical canal, and those that do are often destroyed by leukocytes in the uterus. Half of the remainder are likely to go up the wrong uterine tube. Finally, only about 2000 spermatozoa reach the vicinity of the egg.
Experiments have shown that freshly ejaculated sperm are infertile; they must be present in the female tract for at least 7 hours before they can fertilise an ovum. During the passage through the female reproductive tract, the sperm undergo a process of capacitation that makes it possible to penetrate an egg. The membrane of the spermatozoon head contains a substantial amount of cholesterol, which toughens it and prevents premature release of acrosomal enzymes. Fluids of the female reproductive tract slowly wash away cholesterol and other inhibitory factors in the semen. Most spermatozoa are fertile for a maximum of 48 hours. An ovum can stay in the uterine tube for at most 24 hours before it becomes incapable of undergoing fertilisation.
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