In my website, comment form will closed after seven days from post published, because many spam comment coming. Recently I’ve change that setting to 14 days from 7 days. At same time, spam comments were also increased. But it’s not almost problem. Almost spam comment are blocked automatic by Akismet.
Although I look through the spam comments box before I delete them, there is some possibility of I overlooked non spam comment in spam box. I don’t know that Akismet is making judge the spam from what the element. (Maybe IP, URL, mail-address, or some of words?)