I have been observing from last 6 months.
One of our site generated 8000+ canonical copies due to mod_rewrite bugs so, on 3rd Dec, 2009 our site got the filter.
from 3rd Dec, 2009 to 6th March,2010 I found Googlebot collecting all 404 pages and marking them deleted by removing "cached" link along with SERP result.
from 6th March onwards, Google started its automatic process for 404 cleanup and we have noted some significant reduction of those 404 pages from Google index.
Today after 6 months, I still can find 400+ those orphan (non-cached) copies on SERP due to which our rankings got affected.
So, it's now more than 180 days since Google bot getting 404 status for those 8000 copies.
At max how much time Google need to cleanup all 404 garbage canonical copies?