From the notes:
When editing character appearance by way of Fantasia, players will have 60 minutes of in-game playtime from the initial usage to re-edit their appearance as many times as they desire.


If you don't like your new appearance, I wonder if you can go back and just revert to original and get the fatasia back for later use. By the sound of the above, you'll have to re-create your original appearance, ending up looking the same, but losing the fantasia.