I found a new site (< 2 months old) with 0 PR that already has over 500,000 pages indexed according to the site: command. When I just do a google search on the domain name in quotes, there are around 300,000 hits but when I search through them, I only get 49 results before I see the 'omitted similar results' message and the 'repeat search with the omitted results included'. So this site is almost exclusively auto-generated and duplicate content so I am wondering why it hasn't been penalized. I am wondering about the timing for a Panda penalty. Does Panda work after the fact? In other words, does a site get crawled and indexed and then the Panda algorithm is applied at a later point?
Firstly there are times that two exactly same pages from the same site can come up in the SERP due to differences in which keywords where targeted when extracting the pages from the reverse index used by Google.
As for low long the Panda penalty takes to apply this very much depends on when it detects that the site is a duplicate. There is no hard and fast rule and as it is algorithmic there is no way at all to predict if and when it will happen.
To your last question does it happen only after the fact the simple answer is yes. Before Google's Panda algorithm can work out if a site violates the rules it first needs to crawl a site and index it. Furthermore before the duplicate content can be detected Google needs to have the original canonical site in its index as well and compare the two which is done after the fact not during crawling as it would substantially delay the crawling process which is the exact opposite of what Google is trying to achieve with all of its updates to the crawling and indexing systems.