I like long hair better. I'd say below the breasts is a good length. I think long hair is seen as more of a feminine trait and I think long hair is great as well because of how many different ways you can style it. It keeps things fresh. Short hair can be alright as well though.
I don't know what 'all guys' like. ask a more specific question, unless you want to go door to door asking everyone what they think of long and short hair. I personally prefer a girl with shoulder or a bit longer hair than that.. as long as it isn't so long that its on the floor or on her ass. To counter, I don't like baldies or the butch cut.
Guys usually like longer hair on girls although there are a few guys who prefer shorter hair. I think how long depends on the girl and the guys specific preferences. I used to have really long hair down to my butt and guys loved it. They would always play with. But guys consider anything past the shoulders long, so it should all be fine.
They usually like long hair.
My opinion: shoulders - too short, below breasts - nice, middle of back = waist length... right? well sorta, I think it makes the girl look sexier... on their breast (top part) most appropriate length, suits all, makes the person look well put together.