This last time, I put a deposit on "first pick of the girls" when the litter was only a couple of weeks old. Then I got to delay making my choice until they were 6 weeks old. I think she would have waited longer for the deposit as I had been on her waiting list for a while. But I wanted to make sure I did get first pick. Then when it came time to pick, I would have taken either one of them -- they were both beautiful.
I wouldn't want to make a sure choice though until they were at least 6 weeks old.