Recently I've opened a new forum, I've deleted my twitter and y0utube account(that I never even use) as it was beating my forum on Google when you type in the forum name. They've been deleted for about a week or two. How long will it take for Google to get rid of the results?
When the pages are completly not reachable anymore it normally takes some weeks with low priority keywords.
Google updates daily, it will take a while for pages to disappear, You should have renamed the youtube account and twitter so it would have updated on the next crawl... I'm sure it take 3 crawls to have a web page deleted from google.