Whenever the baby wants
There is no set time when a baby will turn head down to prepare for birth. Most babies will flip up and down multiple times before settling into the head down position. Some babies will spend most of the third head down and some wont turn head down until the day of birth. There are even tricky babies that are head down for weeks and then turn into a bad position when labor starts.
When it comes to the position of the baby the baby has all the control. There are no expectations or norms when it comes to their position.