How long does Mobile Safari keep read items of Reading List in cache?
You have to remove an article from the reading list to purge the contents.

I suppose there is an implied storage limit since you would run out of space on a device once the cached pages exceed the local storage capacity and/or the capacity of iCloud to manage the content across devices.

I haven't tested it, but iOS has the ability to mark some content as cloud backed / temporary and these articles might get culled if you reach a low storage watermark on iOS, but I've never witnessed that happen even on devices where I've run quite full.