We have already wrote about Google webmaster tools in the list of webmaster tools. Using Google webmaster tools you can set up the crawl rate for your website, which increases the traffic for your website. There is no measurements like number of request and seconds between requests. only three options are there faster, medium and slower. 

Now Google Webmaster tool has a new facility, slider to set slower or faster crawl rate and also shows up requests per second and seconds between requests.

To change crawl rate choose your domain in dashboard and select settings. After setting crawl rate it will get expired after 90 days.

Visit : www.google.com/webmasters/tools