I have a phpBB2 forum - heavily MODded against SPAM bots -
It has been 1,354 days since the last SPAM Robot successfully Registered/Posted to this forum.
In that time, this forum has blocked over 22,724 Registration and 33,203 Posting attempts by SPAM Robots.
So you can see they don't stop trying.
My forum is extremely quiet, so same with me, the majority of requests are bots.