well you have to figure out why you are taking fighter levels. Usually, its either just to get some extra hitpoints so your mage survives easier, or you want a fighter/mage who can go into battle with dual weapons chopping things up and casting powerful spells.
If you just want health probably level 9 is best, you'll get your max fighter hitpoints and can then go casting, the level or 2 you will miss out at max level won't really give you much magic to worry about you will still be a casting machine. So 7-9 good range.
If you want a fighter mage, then 13 is the magic number to get maximum fighter attacks per round, boosted by your mage spells, specializations, and what ever else you can dump on. This will still let you become a great caster, and be at the maximum of your fighting potential. Dual weapons blazing, contingency triple sunfires to blast off should your health drop enough, and what ever else your fighting/casting character wants to do.
I always go with dual weapon fighters for all characters, melee combat is just to easy with the powerful weapons you can pick up. You still have an arsenal of spells that you can use as any other good mage.
So just decide what you want your mage to be. A mage with good hitpoints, or a mage that is as powerful as other warriors up close and personal. This applies to other classes as well. fighter/clerics for a dual Warhammer thor, a fighter/thief for a dual chop chop threat, you get the idea.