particleman - 2:37 pm on May 3, 2013 (gmt 0)
A site I developed got hit for the penguin keyword stuffing portion after April 1. I was fairly confident about it because the site tanked for the main term in SERPs. ~1,500 pages or so. Many long tail pages with unique content also were demoted. I gutted the content and rewrote many titles and checked keyword density on nearly all the pages. After doing so on around May I waited. It wasn't until Nov-Dec 2012 that these pages began to appear on their old SERPs. Some were totally removed and just magically came back. The site is now page 2 for the main "stuffed" keyword and climbing. So there you have it. It takes a REALLY long time to overcome this even if you make the proper changes NOW.