A pair of studies done in 2011 and 2012 on password length and construction showed two things: first, customer frustration increases significantly with complexity, but less so with length.

Length is more important than complexity when it comes to secure passwords - and here's a $100 wager to prove it Topics Spotlight: New Thinking about Cloud Computing ...

As for minimum password length, the suggestion doesn’t mean that you shouldn’t consider setting your minimum password length to greater than 8 characters.