If a page on your site is in Googles cache and the page is later removed from your site. How long till it falls out of Googles results (if ever)?
Depends on how often your site was being crawled by Google, and even then I've seen spiders come and go but still the 404's remain. Not sure if a Google sitemap would help this? I also noticed on Google Webmaster Tools you can identify 404's, maybe you could check that out too
So they only last until the bots come back. I have heard google holds onto pages between 30 and 60 days.