The duration is scaling with the player level.
Once purchased, you will receive a Horde Loa-Touched Warscroll and Alliance Tidesages' Warscroll version of this trinket in your heirloom collections. Thus, should you choose to upgrade the Horde version (to level 90), the Alliance version will not be affected. In other words, you will have to upgrade this trinket twice if you wish to use it for both factions. Now that's a lotta mulah!