XF 2.1 Proxied image refresh time/ lifetime settings


Hello, I currently set the image cache lifetime to 0, because of that the cached_image folder size becomes bigger and bigger very fast forcing me to upgrade my server storage after a shot time. That's why I'm thinking about setting a limited image cache lifetime, let's say 15 days, and the image cache refresh of 7 days. So now if an image is deleted within 7 days (or the link to it is expired, for instance in case of Facebook hosted images), it won't be (cannot be) fetched again and my forum will continue to use the older version of the proxied image. But I don't know what happens after 15 days, will that proxied image be deleted from my server or has it been marked beforehand as not-to-delete and will remain accessible? Could you tell me please!

Thank you!