Classic length means that your hair covers your whole behind, with the ends reaching to where your thighs begin.

If someone looks at a woman with classic-length hair from behind when she´s standing up, her entire body plus her behind will be covered with hair; he can really only see the woman´s legs.

If your girlfriend had classic-length hair (already), the seating position in which she is shown in the picture would actually mean that she would either sit on her hair or would have it flow away behind her onto wherever surface she sits on (or if she took it to the front it would flow past her breasts and waist and curl on her lap/between her thighs).