[Bungleau]

I prefer to switch characters to a new class somewhere around level 13, when they've mastered at least one spell school. The exception is warrior: a warrior should shoot for Barbarian ASAP, and then follow the other rule.

Note that if you use spell books to gain a spell, you don't gain the mana boost that you do when you raise levels. If you play it smart, you can actually get just about every spell naturally by changing class appropriately. Three classes times 13 levels equals 39 spells available for the selecting!