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''.
For mages, the hardest part is till about lvl 10, than it will get easier. Most important part is to join the guild, therefore you have to solve the 4 Enclave quests, 2 are easy and 2 are a bit complicated. After you get that, it will be much easier since you will get access to the equipment and skills.
I prefer the summoner build, so my recommendation is to get fire elemental as soon as possible. It's enough to solve 2 Enclave quests (living ice and merple) and give the wizard in Kingsbridge a ruby and a white wolf pelt, that gives you enough reputation for fire mastery.