Oh, I have no clue, but I just assumed Lv99 was the max level. And Adamantoise is not hard, his massive body leaves him incredibly open, and it just takes an insanely long time to kill him. I was Lv63 when I did it, took me nearly 3 hours. It was a spur of the moment thing, and I just didn't feel like doing it later. But yea, all you need to do is just bring a lot of Potions/Elixirs/Downs just in case, because there are times when you just get lazy and he manages to hurt or even kill you. If you stay focused, you can probably rely mainly on Ignis' Regroup skill, but hey, it never hurts to be prepared.
So yea, if you got time to kill, you can definitely do it now. But hell, even if you do it much later, it's not like you're going to make it some 30 min. thing. But of course, a shorter fight is not a bad thing at all, so up to you.