How long would it take to crack an 8-word brain wallet? Assuming my private key for a bitcoin wallet is derived from SHA256(passphrase) and the passphrase is 8 words long, how long would it take the average attacker to crack my bitcoin wallet through a bruteforce dictionary attack? 

Assume there are no other characters besides letters.
A very, very long time.

Oxford English Dictionary contains full entries for 171,476 words in current use.

If you use only lower case letters and they are mostly random words (no phrases - "four score and seven years ago" is like having your password be "secret"), there are about 7.47e41 possibilities.

At 50 million attempts per second, it'll be about 4.74e26 years.

The current estimate for the age of the universe is 1.2e12 years.

If you reduce it to just the most commonly used 1000 words, at 50m gueses a second, it's reduced to 634,195,839 years.

That's pretty secure for a password.