What about a godiva like silhouette in the posture of throwing away scissors with a line that says "go long" ?

I think the length markers would look cool, but I would need a floor length dress for that!

I like the idea of a shirt with a line on the hem that says "if you can read this, my hair is up" or just one that says "my hair isn't long enough"

"Some like it long" made me laugh!

A shirt with an arrow pointing up that just says TLC from the LHC maybe?

I also liked "long hair is my tribute to time" and "you think I need a haircut? ok, which hair should I cut?" "I'm the mane attraction" is pretty funny too.