A Google Webmaster Help thread has a Googler claiming that if you use the remove content feature in Google Webmaster Tools, reincluding that content takes less than a few days.

Google Webmaster Tools has a feature under Site configuration > Crawler access > Remove URL that let’s you remove your site or URLs of your site from Google. If you do this, Google allows you to reinclude that URL within 90 days of the request to get the content back within Google. This is called reincluding content and Google said:

On that page, Google said “requests are usually processed within 3-5 business days.” But the Google Webmaster Help has Jack from Google saying it “rarely takes more than a few days for the site to back in the results.”

Forum discussion at Google Webmaster Help.