One or twice in a playthrough if you're unlucky is my best guess. I haven't seen any real triggers like the fire spells in Disgaea D2, so who knows, you may be totally fine. Best thing to do is to exit the dungeons often and save. Taking a break won't harm your chances of beating a boss and the items you can get from a high bonus gauge (think Disgaea) are rarely worth it. In other words: this game fails as a roguelike, don't worry about playing it like one.
I just started the final chapter. I have to say the story grew on me and actually made the game somewhat worthwhile. Sure, Metallia is cruel, but the point seems to be that the world she's trying to conquer is even crueler.