Rear-facing is safer as long as the baby still fits in the seat in that position, so you turn as late as you can, only when the car seat is outgrown in the RF position.

Signs that it is outgrown are either 1. you reach the weight limit on using it RF (which is 9 or 12kg depending on seat) or 2. the child is too tall for it. Too tall for RF is when there is less than 2.5cm of hard shell ABOVE the baby's head.