It should gradually trend towards using the vast majority of the free space. The disk cache won't consume memory unless you're reading new, not-already-cached pages from disk; so if you're not reading enough data from disk, then it might not consume everything. (E.g., if you've rebooted recently, and just haven't read enough from disk to consume everything.)

If you work w/ removable media, IME the cache will free the pages associated w/ that media when it's removed (as the cache must be assumed invalid when/if the media is plugged back in). Playing a DVD, and then ejecting it, would often result in a large drop in disk cache. (Back before Netflix…)

Only a very small percentage is kept "free". Everything else should be made available to buffer cache or the application, etc....