The cost of Will

Lindon was already mentally exhausted just from bringing himself here, but as he understood it, willpower wasn’t like madra. He didn’t have a finite amount that could run out. As long as he could concentrate, he could keep fighting.

Bloodline (Cradle Book 9)

Lindon needed to balance the playing field.
His head was pounding, and he found it hard to concentrate. He had pushed himself to the brink already in this fight.
He had enough experience now to understand something about how weaponizing willpower worked. He didn’t have a finite amount, like madra or soulfire. Rather, it was more like physical strength, in that by using it he exhausted himself.
And he had already used as much of his will as he ever had.
But that didn’t mean he was out.

Reaper (Cradle Book 10) [By late game, he can fight for quite a long time, so this isn't really a relevant limitation with the length of scenario fights.]