fathom - 4:38 am on Feb 8, 2013 (gmt 0)
As I mentioned in a post above I went through GWT reported links and disavowed the spam three days ago.
Today all of the disavowed links are gone from my GWT and there is no change in traffic, positive or negative.
What there is, however, is a whole new set of spammy links appearing in GWT that weren't being reported a week ago. Also curious is that NONE of my good links, the ones I know about from high ranking pages, appear in GWT.
It appears that Google had already neutralized the effects of the links I disavowed or their impact was minimal.
It was reported as a bug by Google (massive drops in shown links in WMT).
You'll never make a change and see any impact in WMT in the same week (rarely in the same month)... adding disavowing is simply forcing a nofollow like attribute to disavowed links... they will always be shown in WMT until they are physically deleted off the resident page.