What is the expiration time for images not frequently accessed on 8ch.net?

I'm trying to do an independent archive of the qresearch breads. While I can still pull down all the old html files, I find that most of the images and thumbnails for the early days 404 out. The ones that still work are mostly frequently used memes. This leads me to suspect that files which are inactive for a specific period of time are discarded to same space.

Its a shame. Historians researching America's second revolution 100 years from now might like to seen that stuff.