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If you want your thief to become an assassin, make sure you get the Raskalion quest from Scanthril.. Also note that when you go into the pawn shop, you will be asked if you want to be an assassin. You only get asked once, so it is safer to have one of your other characters hold the assassin dagger until the appropriate time.
In terms of when to level up, I don't think your thief needs to wait so long. For your thief to advance from level 15 to level 30, you will need around 2 million experience points.. My recommendation is to change roles once your thief has level 6 in pick locks probably around level 10. He (she) can change to a bard or a ninja (or both) and pick up the moon and fiend spells. I would change roles once they reach level 10 or so (only need 100,000 exp or so) and keep the assassin for the rest of the game (with a party of six, your assassin will probably be around level 40 by the end of the game [assuming you are not too experience greedy or evil!])
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