The main game is fairly easy though it has more Wake Up Call bosses than previous entries if you are not doing side missions. Grinding isn't much of an issue as long as you do the Mark Hunts, not all Mark Hunts can be completed as soon as you get them but doing what you can will net you levels, gil, and good equipment that will help you get through the game. Sadly, once the bulk of the optional quests unlock, completing them will make you drastically overpowered and you can curve stomp most of the games story bosses but that's nothing new to the series...

It can be a very long game, it really depends on how much of it you do, it's got a lot of areas to explore tons of quests and side missions, and the world is pretty huge. So expect 60 hours minimum to beat, over 100 if you plan on doing everything...