If you look at http://aurora2.pentarch.org/index.php?action=stats at the bottom, you'll see various stats, posts, new users, etc. From 2011 to 2015 there was around 1000 new users per year. 2016 had 11.5k. Not all of these users were approved. But that is definitely when the spam picked up pace fairly hardcore. January's numbers for this year were shaping up to follow suit. But that's when I put in the plugin.
Since then, there have been maybe a dozen new users and 1 or 2 guest posts.
Just as an FYI, I implemented a new anti-spam feature. In the last week, this feature blocked 710 spam attempts, both bot registrations and guest posts. It does make my life a lot easier