While most of the posters suggest a reasonable caution approach, which is prudent, my personal experience was different. My girlfriend had her hair just below her shoulders when I met her. She would notice I'd play with it often. Having a long hair fetish myself, I couldn't help it. She asked if I liked longer hair on girls. I said I did. She went with the classic argument of too much work, maintenance, brushing, etc. So, instead of trying to compromise a length, I dove head first and said I'd help her take care of it if she did decide to grow it out. Now, it's mid back, on its way to butt length, and I'm getting all the hairjobs I want. She even comes to me and starts then herself because she knows I will happily wash the cum out of her hair afterwards and give her a nice scalp massage while I do. Which then has it all nice and clean for the next round. ;)
My point is, caution is all well and good. But no risk, no reward. Perhaps offer to help with what she considers the "tiresome" routine of hair care. When you takethe weight off her having to care for it, it helps to absorb the blow of you asking for longer hair. Hell, she might even come to look forward to you doing whatever with her hair. Mine sure does.