They can withstand up to 40kg (not that you'd leave them in their that long LOL). DS1 was in his until he was 23 months old . He wouldn't sleep anywhere else! DD is 8.5 months old and still has her day naps in it. My sister used hers for her DS until he was 18 months old. So basically you can use them for a long time if your bub is happy in there!