The actual length of the passphrase chosen is not stored anywhere.  The actual passphrase chosen is not stored anywhere.  Therefore, you are not going to be able to find out the length of the actual passphrase chosen without knowing what the passphrase is.

The allowable length of a passpharase in Bitcoin Core is much longer than you are likely to have chosen (Passphrases longer than 100,000 characters long are acceptable).  Therefore, knowing the maximum allowable length is not going to be helpful.