How long?

It depends on many factors:

Length
Randomness/entropy
Hashing algorithm
System salt
User salt

Whenever hashing passwords, it is imperative to choose an algorithm that is resistant to hardware acceleration.

Fast hashing algorithms that are insecure: MD5, SHA1

Fast hashing algorithms that are secure, but should not be used for passwords: SHA2, because it is easily accelerated.

Slow hashing, secure algorithms that are resistant to hardware acceleration: bcrypt, scrypt or PBKDF2

MANY sysadmins are epic failures when it comes to getting these matters precisely correct.