image-cache-eviction-delay

image-cache-eviction-delay is a variable defined in `C source code'.
Its value is

300

Documentation:
Maximum time after which images are removed from the cache.
When an image has not been displayed this many seconds, Emacs
automatically removes it from the image cache. If the cache contains
a large number of images, the actual eviction time may be shorter.
The value can also be nil, meaning the cache is never cleared.

The function `clear-image-cache' disregards this variable.