I think the advice has changed on this recently, I was told by my HV that he shouldn't be in it for longer than an hour at a time. This seems very short. He is 4 weeks old and not great at sleeping anywhere else, the movement of the buggy or the car also seems to play a part. He will sometimes sleep in the sling after a feed but not for long and seems more squished up in there than in the car seat.
Any advice/experience greatly appreciated!