I remember the SE update said that you could turn it monsters once per game day (every earth hour.)
Specifically:
"A limit will also be imposed on the number of points that can be earned in one Vana'dielian day (one Earth hour). Even with the above conditions in place, obtaining triggers under this new system should be relatively pain-free, especially when compared to fulfilling the elaborate requirements of some existing NM battles."
http://www.playonline.com/pcd/topics/ff ... etail.html
So it's probably a limit of 10 turn-ins per Vanadiel day, not earth day.
XI Wiki

