This is questionable. If I enter 15 random alphabet characters, it gives me 49 years. 15 numbers is 2 days. 15 characters of punctuation is 743,000 years.

If the cracker was trying to brute force a password, then they would have to try every available character in every valid position. Unless I'm misunderstanding something, and n-length password would take a similar amount of time to guess as any other n-length password with the same restrictions. The cracker doesn't know going into it that I only have letters in my password, so he has to use any valid character.

Perhaps a more useful system would be one that would let websites enter their password restrictions and tell them how secure their users' password can be with those restrictions.