I'd say around shoulder-length; longer is nice, still, up to any length (within reason; wouldn't look good if a woman was sweeping the street, with her hair...), as is a little shorter, but not too much shorter; short hair isn't attractive, on women, in my opinion; it's just too masculine. It must be a burden for women to have to manage long hair, but it's worth it, from the average guy's point of view; I've met very few guys who've said they like short hair on a woman, and, in my opinion, short hair typically takes a good three or four marks off a woman's 'attractiveness rating', out of ten. Still, women don't exist merely to attract me, so they can have their hair whatever length they want.