I said before it took a while. Let s talk about my own experience. I deleted 400 tags from a website. The tags URLs appeared as crawling errors in webmaster tools. It flushed 10 tags every day. In other words, it should have taken 40 days to flush all the 404 errors. I stopped the issue by doing a 301 redirect to homepage. By the 404 is like 410. I takes a long time to get rid of those 404 or 410.