Docs feedback: CI/CD Caching: when the cache is cleared

Link to doc: https://docs.gitlab.com/12.10/ee/ci/caching/

I feel like this doc should go a bit more in depth with when a cache is cleared.

There are several notes about clearing the cache or when a cache may not be available, but I can not find the information when a cache is actually deleted (except from manually clearing it) anywhere on the internet.

There is one section which tells me the cache is cleared when changing cache:key, but it sais "In the next run of the pipeline, the cache will be stored in a different location.". As a system administrator I am therefore wondering if constantly changing the cache:key parameter, e.g. by using the commit SHA, will cause the runner to take more and more space for saving the different caches, until someone manually clears it.

Are these caches cleared from disc automatically at some point?