We only like long hair on a guy if it looks good on him. For example Thor and Loki from the movies have long hair. They look good with long hair (it's not the only reason why they are loved) but you get the point.
If you have long hair ask close friends and family if it looks good on you. Or if you plan on growing your hair out get a wig or hair extensions. Something so you can judge for yourself if it looks good on you.
I don't typically find it attractive, but I have a friend who really does! She says that if a guy can pull off long hair and still look masculine, then she finds that hot. It depends on the girl I guess! Also, Jared Leto is hella fine so as long as your hair is not gross/ matted/ oily then don't worry about it imo.