There is no real caching on IIS but you can configure caching settings in IIS. When you create a new Web application, SharePoint Server 2010 automatically creates several virtual directories that map to folders on the Web server physical disks. When it creates the _layouts/images virtual directory, it adds a caching directive that applies to the whole directory and its sub directories.
This type of caching does not use the caches on the web front ends, but allows the files to be cached in the browser cache of the requesting client. The next time that the file is requested, the browser uses the copy of the item from its local browser cache instead of requesting it from the server again. This reduces the page load time.
The default invalidation period is 365 days. So it means that files in the _layouts can better not change often. But anyway, the files in this folder are deployed during SharePoint installation or solution deployment and this is something you don’t plan to do each day.
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