I think some of it depends on age and style but generally I think long hair should be kept BSL and not too much longer than that. It's very hard to do much of anything with it and tends to just hang. I think everyone's hair curly or straight needs to be styled somehow. I remember on What Not To Wear, Nick said that he thinks long hair should be kept at BSL. I think I agree.
I keep trying to grow mine to BSL but then my stylist wet cuts and it shrinks a couple of inches, but then I've noticed that it looks good at the length she cuts it to so she must know what she's doing.
If I could grow mine out I would donate it to locks of love. It's such a good cause! I met a curly girl whose hair was to her waist and she cut it all off and donated it. I thought that was really cool.