Minimum length of secret key

One rule in cryptography is to use the same key only once. But SRS (or SES)
uses the same secret key for many different email addresses. So the secret
key could be guessed, if it is too short.

Therefore my question to the crypto experts amongst you is:

What is minimum recommended length of the secret key used to generate the
cryptographic hash so that the secret key cannot be guesses by cryptographic
analysis of email addresses modified by SRS?

Roger