I don't think it technically takes until ovy. Females can retain sperm for a while (at least a year) and I believe ovy is when the follicles they develop are moved lower in the body and fertilized with the retained sperm and become eggs. As aalomon said, Ovy is when you can start counting the days until she lays. Unless you have an ultrasound machine or are really good at feeling for the follicles and can guess their size, even then I don't think you can get an exact date.