"What's the overall time-frame I should be looking at considering the drop started on 5th September?"
I don't know. All I can do is relate my experiences as I did in the previous post.
As tedster said, the most important thing to do is to fix any problems a human vistor might encounter (i.e. broken links). It's not a big deal if Google can't find a page that no one would try to visit. But, if Google finds broken links on your site, it makes sense for them to be less likely to send people to it.