Fighters need 3,000,000 xp to attain 20th level; 250,000 per level after that. Mages need 3,750,000 for 20th level, and 375,000 xp per level after that. Needless to say, 25th level would require 4,250,000 and 5,625,000 for fighters and mages, respectively.
When to dual-class is, of course, up to you, but I wouldn't wait that long to switch. Were I to dual a fighter into a mage, I would do so at about 12th level. This allows for a good hit point base and a couple of weapon specializations to be picked up again after regaining your fighter abilities.
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An intelligent mind is the key to the universe.
The most distressing thing about practicing magic is the fact that even the most learned sorcerers cannot accurately describe what magic *is*....
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