I would like to know what the default max-age is of an image or css file, but can't seem to find it anywhere.
Someone suggested it was only 300 seconds, but that doesn't seem to be correct.
The default is infinite.
As newer files are added to the cache, old ones are purged to make space. The file won't actually stay in the cache for ever.
The purge will cause the file to be reloaded if it is called for again.