If concentration is maintained, Spirit Guardians has a duration of 10 minutes! The cleric in my party cast it and sprinted around the zombie dungeon absolutely massacring just about every hostile in the whole place. They were all in groups of three to six, almost all of which would expire to the first hit. He ended up doing hundreds and hundreds of damage in total before the spell expired. He happened to make the two or three concentration saving throws he had to take during that time, but still.
My AOE wizard had just gotten the long-wished-for fireball, and I felt extremely under-powered. The cleric asked me at one point, "why did you waste a whole 3rd level spell on those ones? You could have just waited for me to wander in."
It might have just been the theme of the dungeon, as unintelligent low-hit-point crowds are Spirit Guardians's best use case, but that is the case for fireball too.