We removed a sub directory with about 30 or so pages many months ago, can't remember now, must've been at least 3-4 months by now. But we're still seeing the 404 errors in our WMT. How long does it take for the 404 errors to stop showing up in google's WMT?
If you deliberately removed the pages, you should never have allowed 404s in the first place. You should be returning 410s. If it's a single complete directory, you're looking at one line in .htaccess or your config file. It's not too late to add it.
There is no known time limit, but 410s seem to go away faster than 404s.