I think it must be more than every 1000 years too, at least in Birgitte's case: Thom says he has identified several stories about women he thinks are Birgitte but with a different name, and some even have the trademark braid. During the AOL she was called Teadra. If much of the information about the AOL is lost, it could be assumed that most, if not all, of the legends from before that time were lost too. From this reasoning it seems that Birgitte has lived 5 or 6 times in the last 3,000 years at least.