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Is there way to find out how long a webserver set the expiration time (Max-time) for image files when you browse a site?

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Yes, you need to look for a Headers that server sends in you response to image request.

Usually you can see, something like this in headers:

Last-Modified: Mon, 12 Nov 2012 13:05:04 GMT

With no specific cache control headers. Which gives you browser opportunity to make subsequent requests with header:

If-Modified-Since: Mon, 12 Nov 2012 13:05:04 GMT

and keep file in cache as long as forever, while server keeps returning 304 Not Modified.

To watch the headers you need any kind of Web-debugger, which can be a browser extension (like Firebug in Firefox) or a separate application like Fiddler.

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In Fiddler, look at the Caching response inspector. This Inspector consults the Expires and Cache-Control headers and if they're missing, computes the heuristic expiration time using the RFC2616 algorithm. – EricLaw Jan 14 '13 at 22:36

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