The update strategies of search engines Google, Yahoo, and MSN/Live.com were analyzed by testing the updates of 40 daily updated pages and 30 irregularly updated pages. It was found that the search engines change in up-to-dateness over the years and that none has an ideal solution for index freshness. A major problem is the delay in making crawled pages available for searching, which differs from one engine to another.
Lewandowski, D. (2008). A three-year study of the freshness of web search engine databases. Journal of Information Science.