Unlike women in the first mortal coil, who go through an estrus cycle every month, women in the second mortal coil go through one cycle in probably ten years.
That's when their life force begins to decline, though. If are young and have a lot more years to live, they can keep a single egg viable for up to 50 years.
But even though they are about to die of old age, the bodies of women are so strong that at most, they will go through a cycle once every year. This is the benefit of cultivation.
This benefit ensures that his wife is always ready for impregnation. He too will be ready, and his spear will be very energetic. So he can say with confidence that one month is enough to impregnate her.
The opposite could be the case if either one of them were at the third mortal coil. This is because at the third mortal coil, genetic isolation begins to occur, and sometimes infertility due to castration can occur.
