Akismet Spam Stats:

From 0 to 1,000,000: 70 days

From 1,000,000 to 100,000,000: 190 days

100,000,000 to 1,000,000,000: 257 days

1 billion to 2 billion: 108 days

Akismet has fried over 10,000 spam comments at The Fight Spot.

Check out the Akismet live stats page for a nifty graph of Ham (legit comments) versus Spam (non-legit / pain in the rear comments).