Q1.
Is it easy to decrypt encrypted wallet.dat by [1] using random list attacking method or [2] analyzing wallet.dat ?
[3]In the past, have there been any cases where your passphrase has been decrypted ?
1. Depends on the strength of your passphrase: number of characters and randomness.
2. It's nearly impossible to decrypt using this method.
3. Well, there are some available passphrase bruteforcing tool like btcrecover; so if the leaked wallet.dat's passphrase is weak, it could be bruteforced easily.
What d you mean by "
your", 2nd-person specific?
Me, never, I'm using a passphrase with long, mixed-case, numerical and punctuation mark characters.
Q2,
What is the maximum passphrase length ?
I have no idea,
But you should check this article about passphrase:
Passphrase GenerationIt got all the information you need.