Hi i would say 2/3 weeks,you can usually notice rounded tum at around 5 weeks.make sure you get some cat milk from vets just in case,i once didnt when i first started breeding and the queens milk didnt come at first ,then she didnt have enough,the kittens were crying a lot but i thought this was normal,it wasnt until it was to late that i realised they were hungry,we lost 2.I always keep some cimicat milk in just in case,its also good for mum for a boost,and useful when weaning onto solids to mash food up with.Good luck!