The Aberrant Dragonmark feat states (E:RftLW, p. 52; WGtE, p. 112):
- [...] choose a 1st-level spell from the sorcerer spell list. You learn that spell and can cast it through your mark. [...]
- When you cast the 1st-level spell through your mark, you can expend one of your Hit Dice and roll it. If you roll an even number, you gain a number of temporary hit points equal to the number rolled. [...]
How long do these temp HP last?