Welcome, Neuron!

Unless she specifically asks you if you have any fetishes, don't tell her you have a hair fetish. Don't make things weird.

Tell her that her hair looks great when it does. If you become more than platonic, put your hands in her hair when you kiss her. Women like that. Tell her how soft it is and how great it feels against your face. See if she gets the hint.

If really long hair is really important to you, do a little investigating before you ask her to grow it out. Has her hair been longer in the past? Does she complain about her hair? Do her mother, sisters, aunts, etc., have longer hair? That will tell you about her propensity to grow it out.

Above all, try not to get too focused on length. A woman with good quality, bra-strap length hair can make a guy with a hair fetish really happy under the right circumstances.

Since you brought it up, what are your other fetishes?