The level cap for Final Fantasy XV is level 120, which takes about 27 million of EXP to get to that level. Even a Cactuar that provides 6000 exp per kill would take a while to max your character to level 120. It's really not that rewarding as the final boss of the game, Ardyn, is only level 58. If you're only aiming for the main story, then we only advise you to grind up until you reach level 60.