In a 75g it will take some time. I did the same thing you did not knowing how big they would get.
I kept mine in a 75g for about 18 months and just recently returned it to LFS
when it was about 8-9". It had already developed the Lyre tail and mostly changed color. They are a beautiful fish, but you need a much larger tank. Mine was extremely healthy, but probably could have done better in a larger tank which is why I gave it away when I upgraded to my 90g.
Now I get to go visit it in the display at the LFS
I still have a couple of pics in my gallery:
To answer your question at about 5-6" the transition will become noticable.