When/how did your baby start napping independently

My daughter is almost 4 months old and has almost exclusively contact napped her whole life. A couple weeks ago we started doing 2 naps in the crib and 2 contact naps a day, but her naps in the crib are only 20-40 minutes long (as opposed to 2+ on one of us), and we have to get her totally asleep, hold her for a while, and then try and transfer her. And sometimes that fails and we have to try again.

When did your baby start taking good length naps in their crib? What did you do to help them get there?