[Jim]

If you want to dual to a thief, then dual at level 24 (4000000 EXP), leaving 4000000 EXP for the thief class to surpass the level of the warrior. This will take you to level 25 rogue.

This gives the kensai 4 thief HLA's (go for UAI, spike/time trap, assassination), and 5 warrior abilities (WW, GWW, GWW, GWW, hardiness).

Go with the thief by the way, since (as has been pointed out already), you can still cast spells, and you can dual later with no repercussions for the second class (meaning you get some warrior HLA's, more HP,more weapons proficiencies, higher kit bonuses and better saving throws). Dualling a late kensai to a mage leaves little room for high level spell casting. In fact, the following effects will happen when dualling to a mage:

Dual at:
Level 7 kensai: Takes you to level 31 mage. Losing no spell slots.
Level 9 kensai: Takes you to level 30 mage. Losing no spell slots.
Level 12 Kensai: Takes you to level 28 mage. Losing no spell slots.
Level 13 Kensai: Takes you to level 28 mage. Losing no spell slots.
Level 14 kensai: Takes you to level 27 mage. Losing a total of 1 level 8 spell slot.
Level 15 kensai: Takes you to level 22 mage. Losing a total of 1 level 8 and 1 level 9 spell slot.
Level 16 kensai: Takes you to level 26 mage. Losing a total of 1 level 8 and 1 level 9 spell slot.
Level 17 kensai: Takes you to level 25 mage. Losing a total of 1 level 8 and 1 level 9 spell slot.
Level 18 kensai: Takes you to level 24 mage. Losing a total of 1 level 8 and 1 level 9 spell slot.
Level 19 kensai: Takes you to level 24 mage. Losing a total of 1 level 8 and 1 level 9 spell slot.
Level 20 kensai: Takes you to level 23 mage. Losing a total of 2 level 8 and 1 level 9 spell slot.
Level 21 kensai (latest possible dual): Takes you to level 22 mage. Losing a total of 2 level 8 and 1 level 9 spell slot.

Looking at the above info, dualling at the "favourite" level 13 results in losing no spell slots whatsoever. If, however, you do want to go for a late dual to a mage, there is no difference in dualling at levels 15 through to 19. You will be casting the same numebr of spells. There is then no difference in dualling from levels 20 and 21.

So, to summarise, dualling at level 13 gives your kensai +4 to hit, +4 to damage, 8 weapon proficiencies, and will eventually produce a mage that can cast the same number of spells as a level 31 single class mage. This level also nets a hearty saving throw bonus. (something like 2,2,2,3,2)

Dualling at level 14 is no good here, since the kensai is getting no more saving throw bonuses, weapon proficiencies or kit bonuses, but will get a +2hp boost over the character that dualled at level 13. This really isn't worth losing 1 8th level spell slot over.

Dualling at level 19 is the next ideal step. At the cost of losing 1 8th level spell slot, and 1 9th level spell slot (large price!), the character will get +6 to hit, +6 to damage, 10 weapon proficiencies, the best saving throws obtainable by a warrior and around +8 HP over the character that dualled at level 13 (these HP bonuses assume the character has reached the XP cap).

Dualling at level 21 is the next ideal step. At the cost of losing 2 level 8 slots and 1 level 9 slot, the character will have max saves, +7 to hit, +7 to damage, 11 weapon proficiencies and 2 warrior HLA's. This character will also have a total of around +10 HP over the character that dualled at level 13.

So, if you want to go for a late dual, I would say go all the way and dual at level 21 Kensai (to get some nice combat bonuses and a couple of WW attacks). You will still be able to cast 3 level 8 and 3 level 9 spells. After level 13, the next most sensible place to dual is level 19, after that, level 21. It's your choice, but don't dual between levels 14-18, since you're losing out on warrior abilities and gaining no extra spell slots.