Has anybody else noticed the annoying habit, of Windows 8 and 8.1 deleting media file thumbnails? I believe it's done every restart, via the maintenance scheduler. Googled the problem quite a bit, tried the various workarounds, i.e. set deny permissions in the explorer menu. None seem to work. I am the photographer/videographer, for my local model airplane flying club. I have many many images/videos stored, and it's really frustrating to have to sit and wait for the thumbnails to reload every time.
I'm not normally using system generated thumbnails so I have no experience over time, but from reading up it seems to happen to so many that it might simply be normal at least for users with a lot of thumbnailed files. You would probably do best to use some third party software to "index" that amount of media files for you. Those keep their own databases and aren't left to the whims of the operating system's more or less undocumented features.
I have however seen some mentioning of a possible registry setting that affects how much data the thumbnail cache can store, but most references are to older versions of Windows and they could easily have changed something for Windows 8. All the good that might do is simply delay the time before the cache resets, but perhaps check out that path if you haven't already.
Also make sure you don't have any "cleaner software" installed that might delete such things on a regular basis. There's so much crap software of that kind out there that does not always tell you all it does.