Google announced a couple of updates this and previous month. First one occurred on September 28th.
New exact-match domain (EMD) algo affects 0.6% of English-US queries to a noticeable degree. Unrelated to Panda/Penguin.
— Matt Cutts (@mattcutts) September 28, 2012
Then another one happened between September 28th and October 5th which resulted in massive algo and SERP changes.
This was another quite big update and many webmasters were confused did they were hit by EMD or algo update. Another update happened October 8th. This is latest Penguin update:
Latest update (so far) occurred today:
Minor weather report: Update of goo.gl/OpIDL launching today. ~0.7% of English queries noticeably affected.
— Matt Cutts (@mattcutts) October 9, 2012
So in total, I think that at least 1.6% of English queries were hit. That’s minimal because we don’t know how much queries were hit in total by Penguin.