All women are born with millions of immature egg cells in their ovaries. Ovulation happens when an ovary releases a mature egg through the fallopian tube. This process of getting an egg to mature will take approximately a month. Normally, only one of the pairs of ovaries will release a matured egg ready to be fertilized. Usually, it will take 12-24 hours before the matured ovum dies when not fertilized. But when they are fertilized, they cling to the walls of the uterus.