Easily under 72 hours. There's a headpiece that will give you extra experience until level 10, Food gives a 3% bonus, Armory gives ~50%, and the Free Company experience gain is 5%. Combine this with the fact that the Armory boost applies to FATEs and I can guarantee you that there will be people who will either be FATE farming or dungeon hopping in a pre-made in order to get their Ninjas to lv50. I would bet that by the time we reach the 72 hour mark we could have Ninjas in full ilevel 100 gear with their 110 Rowena weapon.