Assuming it's in line with all the other powers, it would last for as long as the sprite continues to sustain it. If we compare it to the spirit power Guard, which does something similar, then it can be sustained at no penalty, and doesn't prevent the sprite from doing other things. Sustaining it would cost 1 task, and would cost another task if you want the sprite sustaining it to change to a different target. It's logical to assume that because Diagnostics specifies the sprite can't do anything else, that's the exception and so wouldn't be the case for Stability.