Fertilization can occur only after the egg leaves the ovary and hits the Fallopian tube. Thus, the woman is on day 14 of the menstrual cycle approximately. Even though sperm can stay in the female reproductive system for up to three days, the egg cell has a lifespan of just 24 hours after it is released, which means that fertilization can take place only within the first 24 hours after fertilization. However, intercourse may have occurred between 2 and 3 days before.
To sum up, the fertilization process can take several hours, although the length can vary from case to case, so giving an exact figure is complicated. Generally, it occurs within 19-24 hours.