CainIV - 7:14 pm on Jul 20, 2012 (gmt 0)
Some notice drops immediately (within two weeks) and others don't. It seems to me that the threshold is becoming more and more refined and customized based on the analysis of inbound links.
Severe examples of paid links and unnatural link schemes seem to suffer significant drops - much sooner, where as websites we have worked through this issue with that had one or two examples of links that might be construed as unnatural, but a host of other great branded links and brand signals, have been mostly fine for months and months.
In the end, they seem to be giving more benefit of doubt to the brand and site with a more natural link profile, and this only makes sense. I would recommend most sites consider the warning imminent and immediately start cleaning up...