If you have Kontakt Player, then yes, the vast majority of Kontakt libraries that are free will only play for 15 minutes before you would need to load them again.
The Player is essentially a limited trial version of the full Kontakt sampler. So that you can check out if you want to purchase the full Kontakt.
However, the creator of a library can purchase a license from NI that will allow the library to work fully in the Kontakt Player. The cost of the license is why there are hardly any fully working (compatible) free libraries for Kontakt Player.
You can use the wav and aiff files available in a variety of other free software products.