If I recall correctly, the game is beatable around level 20, and all levels past that are considered ''heroic''. If you go around completing all quests, exploring all areas, etc, gameplay can be more of 100 hours total.
Mages are definitely hard at first, but in my experience with mages, they start to get good around level 7, and really great around level 13. But I don't think there's a specific level where they become either of those, just depends on what you mean by ''good''.