Lorel - 10:09 pm on Dec 24, 2012 (gmt 0)
I was hit by panda #9 or 3.1 November 2011 and lost a major portion of my traffic at that time. I didn't figure out it was overoptimization until April when I started deoptimizing my pages (i deleted several that were not important and added text to several more that didn't have enough). I added new pages to several sections of my website and also set up a Wordpress blog on my site and started adding pages about 2-3 per month. I continued with the above every month until now, which means about 15% of my pages were updated and deoptimized since April.
The lowest traffic was in July (usual summer slump) then it started slowly increasing every week. I had a large increase in traffic with Panda 21 Nov 2012, with the highest peak in early December (then the usual seasonal slump for the holidays -- ranking is rising but traffic is down).
I believe there is a threshold of a certain percent of pages that get updated or deoptimized. In my case it is around 15% but may be different for others. I'm still not back to 2011 traffic but still intend to keep working on my site.
BTW, I also have a couple pages where I need to repeat the same keyword over and over for visitors benefit so I used images on those pages instead of h2 tags.