There was a question on Information Security (Stackexchange) about a year ago which was exactly adressing this issue:

https://security.stackexchange.com/questions/92512/how-much-...

This was a follow up of a more general and well received question (How critical is it to keep your password length secret?), where some of the answers imho are a really worth reading:

https://security.stackexchange.com/questions/92233/how-criti...