I've recently had a problem with my WordPress installation which was caused by the Database hitting a hosting limit of 150Mb which pretty much locked up my site, preventing post update, new real comments, the upgrade to WP3.7.1 etc.
I've noticed that typically in a month I was getting 3000-5000 comments accurately flagged as spam by Akismet v2.5.9. In October 2013 this increased to 42,000! and the DB hit the 150Mb limit.
I've had the DB size limit lifted on my website and could delete all the Akismet spam but it appears like the number of flagged spam in November is heading the same way (over 4,000 in the first 2 days!)
I have the Akismet plugin set to delete comments after 30 days but to be really honest I'd be happy to have an autodelete after just a day as I never check the spam messages as it is remarkably accurate at flagging the spam.
Any help will be gratefully received