Q: How long would it take to crack an 8-15 character Wi-Fi password?
A: If we take 15 characters as the longest the password could be, that gives a 62 character search depth (26 a-z+26 A-Z+10 0-9 possible characters). Doing the math on the permutations of all possible passwords up to 15 characters using any combination of 62 characters, comes out to be around 7 x 10 to the 26th power possible passwords.
If you have access to a server farm capable of 100 trillion guesses per second, it would take over 2 thousand centuries to crack (statistically speaking. You could get lucky and guess on the first try).
On your cellphone at 0.5 guesses per second? Fugetaboutit!