Google routinely makes Cache copies of web pages. How long do you save
these web pages before you update them, especially chat room web
pages?

Hi! Thanks for the question.

After doing some research I found out that Google updates its caches
between one to 2 month periods for any type of web pages. The article,
"The Google Cache Explained", discusses the matter in simple terms.

"If you update your web site, Google will still have the older
snapshot of your web page in its cache until it spiders your site
again, and updates its cached snapshot. Sometimes, it takes 2 months
for Google to update its cache. If Google spiders and caches your web
page today, and tomorrow you make modifications, users finding your
web site in Google will see the old cached snapshot, not the new page.
However, if a user does a search on Google, and finds your site, when
they click on the URL, they will still get the updated, "live" site.
It is only when they click on the 'cached' site that they will see
these older snapshots."

If you would want a fairly in-depth discussion of Google cache
updating you should read something about the Google Dance. It is the
time when Google makes an update of its index. You could read it here:

“Google Dance - The Index Update of the Google Search Engine”
http://dance.efactory.de/ 

Previous Google scheduled updates are provided here by the Webmaster
World website.

“Google Update History”
http://www.webmasterworld.com/forum3/2657.htm 

So you can see that Google updates its index and cache in about a
month or so and 2 months at the maximum.

Search terms used:  
Google cache update
 
I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
 
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Regards, 
Easterangel-ga 
Google Answers Researcher