Nanahoshi comes at a rate of about once per month and uses the bath.
It seems she really wants to come more often, but it seems she's showing restraint.
I'll just say it to prevent misunderstandings, but I've never tried peeking in on Nanahoshi in the bath.
I don't know if Nanahoshi is being cautious about that area as well, but she only ever seems to come by at the times when Sylphy is around.