I've always assumed that there were different types of flags for human review - and they would come with different degrees of urgency.
1. The most basic is the "army of 10,000" that get assigned a specific SERP to review, and if your site has a URL in that SERP, you'll get a human review. Not very high urgency and more about the SERP quality as a whole, rather than "just you".
2. Next might be a flag that says "the crawl just found significant site-wide changes." I don't know for sure that this exists, but recent reports of ranking drops in some (but not all) cases of site-wide changes make it seem like a possibility.
3. Up one more on my theoretical scale of urgency would be some automated flag that gets set because of things like "this domain just got a big jump in SERP impressions". That's got to be more urgent and take place relatively fast - and it would be about "just you". A similar flag might get thrown by an uncharacteristic surge in backlinks.
4. Even higher on the urgency scale might be a flag set by a specialized crawl of some kind - looking for possible cloaking, or tracking back from links that are related to an identified link seller.
As to what the time frames are, I could only take a shot in the dark - but I'd guess that the most urgent might happen within 24 hours.
Does the system temporarily reduce how often it shows your sites pages in serps while a review is pending?
I never thought about that, but with all the evidence recently of some kind of "dayparting" in the SERPs, it does sound like a possibility.
Now the heck can we verify any of this conjecture? Seems like it would take industrial espionage, finding some "loose lips" inside the Plex, or something similar.
[edited by: tedster at 6:08 am (utc) on Jul 6, 2010]