About 6 weeks - I think Annika was somewhere around there, but it was so gradual I'm not sure exactly. Somehow she went from staring at us with a serious look all the time to grinning and cooing, but I'm not sure when! She's most smiley in the morning, too.
The cooing is so sweet
I swear she deliberately repeats the word "hallo" (along with saying "go", "humboo", "ungie" and that sort of thing.