i don't remember the numbers at all anymore. but i'll try to help ... there are two stats that you're going to be interested in extra attacks from weapon proficiencies and the extra ac bonus that swashbucklers get every few levels.
That said, my recollection was that swashbucklers are best dualled to the wizzie fairly early, because there's no particiularly great gain from lots of extra swash levels.
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Either 9 or 13. 9 gets you an extra half attack and since you are not too high level yet, it doesn't take too long to get your fighter levels back. 13 gets you an extra half attack on top of the one at 9 for a full extra attack per round, but the downside is that 13 is getting pretty high level so it takes a long time to get the fighter levels back.
Just looking at the tables:
Fighter 9 requires 250,000 experience. Wizard 10 when you get the fighter levels back requires 250,0000 experience too, so you only need 500,000 total experience to become the powerful kensage.
Fighter 13 requires 1.25 million experience. Wizard 14 requires 1.5 million experience. Here you need a huge 2.75 million experience total to get the fighter levels back....
Going above level 13 before dualing is definitely not recommended.
The other guy said level 16 is the best time to dual. Well look at this:
Fighter 16 requires 2 million experience. Wizard 17 requires 2.6 million experience. 4.6 million experience total. You will be in Throne of Bhaal before getting your fighter levels back.... definitely not good.
All of the above is for the kensage though. Swashbucklers work a little differently. I have never played a swashy so I don't know what the optimal dual-level is, but just remember that the higher level you become in your first class it takes exponentially longer to get those levels back.
Swashy's should dual at level 10 or 15 as those are the levels the kit gets its THAC0/damage/AC bonuses. Proficiencies don't matter as Swashy's don't get the bonus 1/2 attack from specialisation.
L10 Swashy (+2 bonuses) takes 160,000xp, L11 mage takes 375,000 - can still reach L30 mage L15 Swashy (+3 bonuses) takes 1,100,000xp, L16 mage takes 2,250,000xp - can still reach L28 mage which gets just as many spell slots as the L30 mage (according to my manual at least). Is it worth significantly more downtime for only one more bonus?
But honestly, if this is your first time through the game, don't dual class. Dual classing is too complicated if you don't have any experience with the game. Go with a Mage/Thief multi if you really want to try out both classes or just pick up one of the three NPCs that are this dual/multi already - in fact, you should do that. Imoen (thief dualled to mage) will cover you for the first dungeon, then go get Jan Jansen (Illusionist/Thief multi with some useful equipment and lots of good banters) instead once you get out in the open.
L15 Swashy (+3 bonuses) takes 1,100,000xp, L16 mage takes 2,250,000xp - can still reach L28 mage which gets just as many spell slots as the L30 mage (according to my manual at least). Is it worth significantly more downtime for only one more bonus?
I would have to say, no, it is not worth the downtime. 3.35 million..... you would be nearing the end of SoA by the time you reach that and get your swashy levels back.
The longer you wait to dual-class, it takes exponentially longer to get those levels back. I would personally never take more than half a million or so experience in the first class.