When did naps in bed/crib start lengthening?

Hi! My baby is 6.5 months old and contact napped exclusively until she was 5.5 months old when we transitioned some naps to our bed but she does 35’ naps almost always if I don’t rescue them except for the very rare 1.5 nap she does all alone sporadically. I wanted to know when did this start to change for your babies and if you did anything to help them nap longer on their own.

Thanks!!