I've played this game a zillion times but this was way back in the 2000s. Problem is I played on different difficulty leveles (I know Hard increases XP) and last time I played was some 8 years ago, so I can't remember this properly. Can anyone tell me how much, roughly, experience at the end of the game your characters will have if:

_Original IWD1 (not EE).
_IWD + HoW + TotL.
_Play on Normal difficulty.
_No mods or tweaks.
_Do all quests and subquests.
_No grinding or XP farming.

I need this since I'll be dual-classing some characters and would like to know what's the latest I can do this, so that he'll regain his original class before the game ends (and have time to play with both classes). Rough number should be enough.

End of Original Campaign plus the two expansions?

Yes. I checked my final save on core difficulty and have about 2 520 000 xp per character (that's after 2 expansions).

Thanks. Does this mean that a wizard on normal difficulty will *never* get to cast level 9 spells (requires 18th level and 3,000,000 XP)?

You mean "gain the innate ability to cast level nine spells", since a wizard can cast a level nine spell from a scroll.
And, supposing your data is correct, mathematically your conclusion is sound. :)

Monster Summoning and Power Word. No big loss.

Well, unless either:
* You're playing the EE, which adds some more spells at that level, in which case it is a noticeable loss.
* Your mage is a specialist who's forbidden to use Conjuration magic. As a result, the character couldn't use either spell anyway, so there's no loss at all.
* You are using a smaller party, in which case the XP figure given won't be accurate, due to XP being split between characters (in contrast to some games, like most (not all!) Dragon Quest games, where XP isn't split), and the increased XP would likely allow you to reach 3 million XP. (If that estimate is correct in the 6-person party case, a solo would get 15 million XP, which may be enough to max the levels of even some multi-class characters (though maybe not triple-class).

* I'm not playing EE (my first point in OP).
* Zero loss is still no big loss (just like, for example, if something is not an animal then it's not a big animal).
* As per above, if my party size or composition allows me to cast level 9 spells, then there is no loss so there is no big loss.