Whether classic length, thigh length, or knee length - her hair looks spectacular and by any means good and long enough for anyone on this forum.
I wouldn´t bring up hair per se as a topic, but I would compliment her on her hair. Nobody reaches such lengths by just forgetting to have it cut. She must be interested in growing it that long.
Don´t give her the feeling that you are just into her hair.
She looks really really nice otherwise, too, anyway.
For me, she would be a dream come true.
Good luck to both of you.