DD1 about 8 months because she was tossing and turning and seemed to be unable to get comfortable all of a sudden. I think she wanted to sleep on her tummy and did that straight away in the cot so the transition was easy.

DD2 about the same but for safety because she was trying to sit up and stand up in it. In fact I left it a bit too long in the hope I could keep her in it and she fell out after trying trying to stand up. Not such an easy transition, but then she wasn't sleeping all that well anyway.