SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Key Management

We recommend creating user keys at least 1024 bits long. Protect your key with a good passphrase. Make it lengthy and use a mixture of lowercase, uppercase, numeric, and symbolic characters. Don’t use words found in a dictionary.

Empty passphrases should be avoided unless you absolutely need to use one—for example, in an automated batch script. [11.1.2.2]