In my experience Googlebot has complied with robots.txt changes virtually immediately. If you don't want the page to be indexed at all cost you can always include <meta name="robots" content="noindex,nofollow"> in the page head.
Use the WebmasterWorld server headers checker [webmasterworld.com] to determine what your Expires and Cache:max-age settings are for your existing robots.txt. Google adheres to these settings reliably (if you provide them).
HTTP/1.1 200 OK Date: Wed, 06 Aug 2003 16:14:01 GMT Server: Rapidsite/Apa/1.3.27 (Unix) FrontPage/188.8.131.520 mod_ssl/2.8.12 OpenSSL/0.9.7a Cache-Control: must-revalidate, max-age=7200 Expires: Wed, 06 Aug 2003 18:17:30 GMT Last-Modified: Sat, 02 Aug 2003 05:40:41 GMT ETag: "1b6eae6-ac6-3f2b4ed9" Accept-Ranges: bytes Content-Length: 2758 Connection: close Content-Type: text/plain
This shows that my robots.txt is to be considered valid for only two hours, and must be re-fetched if the user-agent has an older copy.
tschild I can't add the meta tags to the page b/c its a dynamic page that essentially performs some database work and redirects to another page.
jdMorgan: I haven't previously set the expires and cache:max-age but I will now.
Here is my scenario that didn't work:
I updated my robots.txt to Disallow a page. Within one day I saw at least one googlebot access my new robots.txt One week later I upload the new page I don't want spidered. One more week later I find it in the index.