| 5:26 pm on Oct 28, 2011 (gmt 0)|
Once fixed you will have TWO sets of urls and the "fixed" version will start to gain history. If you 301 the old to the new and/or robots.txt block the old, and possibly even perform a removal request if you take the robots.txt route, you can be assured that the new will outrank the old.
That being said the old url's will need a good long time to fall out of your link graph, even if they don't rank in serps. Google forgets nothing.
| 5:38 pm on Oct 28, 2011 (gmt 0)|
I would 301 redirect the old URLs. You want to retain the traffic for at least as long as other sites are linking to the wrong URL and certainly while the old URL is listed in the SERPs. Second best is to return 404, or meta noindex. Worst way is using a robots.txt disallow.
It will take at least a few weeks, often a few months for the SERPs to be cleaned up. It may take 6 months for the URLs to drop out of the WMT reports. That is NOT a problem.
| 5:46 pm on Oct 28, 2011 (gmt 0)|
internally your traffic flow will correct in a few days. From a search webmaster tool standpoint it will take 30 - 45 days to clear out the errors.
Be patient with those, google webmaster tools sometimes is like watching paint dry
| 7:18 pm on Oct 28, 2011 (gmt 0)|
It also depends how many pages on your site had the error and for and how long the errors were on your site.
If you have a major error that affects Googlebot crawl on every page of your site for a month, it can take a year to fully recover lost traffic.
| 7:28 pm on Oct 28, 2011 (gmt 0)|
It takes a while. On October 9th my website went down for an hour, and the WMT still shows me the network unreachable messages.
Be patient, it will eventually happen.
| 8:52 pm on Oct 28, 2011 (gmt 0)|
I think it depends on overall frequency of crawling rather than calendar time. I have one directory that I created in January, with 301s in place at the outset. Google still crawls the old URLs about twice a month. I know because I've been waiting for them to stop so I can clean out my htaccess. The last time I did this, msnbot or someone like them came knocking at the door within days-- literally-- after I'd decided everyone had accepted the changed URL so it was safe to drop the redirect. Grrr. But now they can bloody well take their 404 and lump it.