Well, what I feel is this won't remove the currently indexed pages so easily. An idea that popped out of me is download your website on your machine, change the filenames and do a simple find and replace and upload them to your website. Make sure you delete the old files.
This way your website pages won't be find. What do you say?
Even though google won't crawl a page that is disallowed with robots.txt, they can still list a link to your site. If you want to ensure complete removal of your site, I would recommend not using robots.txt, but rather using meta tags on each page. Let the bots crawl the content, but don't let them index it. This keeps google from listing links to pages you don't want it listing links to.
I think it should be much sooner than a month. If it were an office pool, I'd bet on less than 10-14 days. Let us know when you start to see pages leaving (it will probably be a gradual process as we try to fetch each page and find the robots.txt).