I think it was about 4 months. They started wriggling around a lot and just looked a bit frustrated as could see much over the sides of the carrycots. They seemed to want to look around when we were out rather than just dropping off to sleep straight away. Also coincided with the weather getting warmer so didn't need to get them so wrapped up and snug in the carrycots.
Hi, The advice with the buggy said not to use the cots after 6 months, or they can sit unaided, which i initially scoffed at having had older singlton ds in a pram well beyond sitting up unaided (secured with a harness), however I found that as they got bigger and heavier, with the way the carry cots snap in, they became less secure if more weight was at one end or the other - they would actually tip up if all the weight was at one end. So in answer to your question, I found that by 5 months my boys were ready to sit up and see the world - although they were not yet sitting unaided - so we put the cots in the attic. Boys were born at 36 weeks, and around 50th percentile. Hope that helps!