The Sperm's Long Journey. A man may ejaculate 40 million to150 million sperm, which start swimming upstream toward the fallopian tubes on their mission to fertilize an egg. Fast-swimming sperm can reach the egg in a half an hour, while others may take days. The sperm can live up to 48-72 hours.Only a few hundred will even come close to the egg, because of the many natural barriers that exist in a woman's body. It takes about 24 hours for a sperm cell to fertilize an egg. When the sperm penetrates the egg, the surface of the egg changes so that no other sperm can enter.At the moment of fertilization, the baby's genetic makeup is complete, including whether it's a boy or girl.he sperm can live up to 48-72 hours. Only a few hundred will even come close to the egg, because of the many natural barriers that exist in a woman's body. It takes about 24 hours for a sperm cell to fertilize an egg. When the sperm penetrates the egg, the surface of the egg changes so that no other sperm can enter.