Max is 8 digits - theoretical limit is 10, but when using it, it unlocks by *both* providing 10 digits, or first 8 only (!).

As for automating brute force of lock code, it's interesting idea. Lock code is trivial to break (DES), but only, if You have access to root filesystem, which *shouldn't* be possible without rebooting, if already presented with lock code prompt. When prompted, every 2 wrong attempts there is delay, that increases in 3th attempt, then disappear for next 2 attempts, and so goes on, in circle. Both delays are customizable (don't remember where, but it is easy to find it, IIRC).

Honestly, I can't think of any way, that would allow "3 letter agencies" and their specialists to retrieve lock code/unlock, without causing reboot (messing with flash storage content directly, without device in middle, should definitely cause reboot, and it's far from achievable, without de-soldering One NAND or cutting motherboard and doing extra-precise connections to certain paths itself, without harming other paths).

If anyone have sensible ideas, it's worth to discuss them in Truecrypt thread
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yablacky, it's great to hear, that You've managed to move lock code prompt! May I ask, what are current pitfails about? Maybe someone will be able to help overcome them?

/Estel