We can do password guessing way better than brute force by generating strings using a Markov model instead of randomly. I wrote a paper on how to do this a few years ago <a href="http://www.cs.utexas.edu/~shmat/abstracts.html#pwd" rel="nofollow">http://www.cs.utexas.edu/~shmat/abstracts.html#pwd</a> Some of the techniques have since found their way into John the Ripper. (and possibly other tools; I haven't checked.) The upshot is that you can't calculate strength with a simple formula like this. At best you get an upper bound on the time needed to crack, by a few orders of magnitude.
The most important passwords I have are the ones for online banking and the like where you certainly aren't going to be doing bloody 25 billion attempts an hour.
What does "Random Alpha/Numeric" mean ? - sequence not readable ?<p>I observed that if you mention your password has random Alpha Numeric the time taken to break jumps to thousands of days.