You can deindex a website just within a few hours.
First thing to do is to create a robots.txt file and upload it on your website’s root.
It should contain
User-agent: *
Disallow: /
Or if you want to deindex just a folder, use:
User-agent: *
Disallow: /images/
After that, login to google webmaster tools, add your website there, verify it.
Then click on your domain from google webmaster tools > index google > delete or emliminate a resource.
Press “hide temporary” and click ok, no need to type anything in there.
That was all, google will just deindex your website within a few hours after you do all these changes. Even if you have thousands of pages indexed.