I want to dual class a Kensai to a mage but...

I don't know what level would be best to dual? Who can help me out with my dualling process?
I will love you long time.

Level 9. You've just got a a kensai bonus to your THAC0 and a new proficiency slot. Next good opportunity is level 12 or 13 but it is too late. You would have to play a pure mage until you reach level 13 or 14 before you obtain a kensai/mage. This would happen somewhere in the Underdark or even later depending on how many people you have in your group.

topic was brought up a couple of days ago,, everyone agreed at lvl 13

<font color="gold">Yes, lvl 13 is a very good time to dual a Kensai. Dont have any good reason, except that by that time, you have already played as the single Kensai for a while, and maybe starts to get bored of it, but then it will become a much better AND more entertaining character when you dual him with a Mage.
But if you dual him earlier then that, you might get a bit bored of him at the end of the game. ;) </font>

A few notes:
1. If your reason for dualling is "I am bored with a kensai" then dual at level 13. If you have more serious reasons for it then level 13 is really too late. You can play a mage from beginning instead and obtain a more useful character than a handicapped mage made from a kensai at level 13. Dualing at level 13 is possible in ToB but not in SoA except a solo kensai/mage. See your manual and add together XP needed for 13th level kensai and 14th level mage. This is the number of XP needed by the kensai dualed to a mage at level 13 before he becomes a kensai/mage. Take into account that each member of a party of 6 will have about 2700000 XP at the end of the game.
2. You will not be bored by a kensai/mage dualed at level 9 at all. First of all, you get a kensai/mage soon and not at the end of the game. Second, your kensai/mage will be more powerful mage because he will have higher level as a mage. Third, his fighting abilities will not be considerably worse than the abilities of a kensai dualled at level 13.

Consider all this before you decide to dual at level 13.

when you dual has a lot to do with whether you have ToB or not. dualling at lvl 13 denies you a LOT of high-level spells, which makes your life really tough, especially if you are soloing or don't have big mage and cleric characters. (dualing at 13 and having only Aerie as a spellcaster doesn't work too well) if you have ToB, then dualing at 13 is ok. with only SoA, the most i would do is lvl 12. i dualed a K-M once at 12, before i got ToB, and it was kinda tough in the middle. i can only imagine how much worse it would be at 13.

However, it really is your choice.