As paws said... these things are strictly based upon the mathematical permutations. The "hardest" password will always be the longest. For example, any 15 letters, all the same case, will come out nearly 20k years (any15 times a single number is only 4 days). Capitalize just one of the 15 letters and it becomes over 16 million years (doesn't matter which one). Capitalize any two, and you are at over 800 Billion years.
If I type my name in (First with the first letter capitalized, space, last with the first letter capitalized) and it says over 117 million years. However, no one would consider that a safe password.
So, when picking a password, the longest one will be the toughest. The variety of "styles" of characters (Uppercase,lowercase, numbers, and special characters if allowed) will increase the security. However, you could have a 20 or 25 character password that contained upper and lowercase letters, numbers, and special characters... but is actually your name and birthday... and it may be considered a "strong" password but would not be a "smart" password. Brute force is based upon permutations, but behavioral analysis is often a faster way to break protection.