If the npc said for every 36 hours people would get confused. They would go is that 36 hours aftwer i do my levequest? or 36 hours n general. Also it would be too much text and harder to translate.

the full term would be "for 36 hours (earth time)". << too much text
then "at a time"

Also it isn;t the devs fault you guys took the term to literally.