I’m assuming that you are using the free Kontakt Player. It’s free because it is essentially a trial to see if you want to buy the full Kontakt version. The Player comes with 50 free instruments from Native Instruments that don’t time out.
Most free libraries will time out after 15 minutes in Kontakt Player as it requires the producer to purchase a license from Native Instruments. And it’s quite an expensive outlay for something people are offering for free as with many of the libraries on Pianobook.
Hope that explains things if you are using the Kontakt Player.