There is not a way for you to set the password strength. iThemes Security uses the WordPress Password Strength Meter to determine whether or not the passwords are strong.
The WordPress Password Strength Meter uses the
zxcvbn library to determine the strength of the password.
zxcvbn is a password strength estimator inspired by password crackers. Through pattern matching and conservative estimation, it recognizes and weighs 30k common passwords, common names and surnames according to US census data, popular English words from Wikipedia and US television and movies, and other common patterns like dates, repeats (
aaa), sequences (
abcd), keyboard patterns (
qwertyuiop), and l33t speak. - zxcvbn