How long does Seismic Wave last?

The damage arrives as separate shockwaves on a 1-second trigger interval, not as one hit. Seismic Wave keeps firing them for the whole 8 seconds, so it pays you back for staying near the target, and it loses value when you move away or the fight ends early.

Treat it as pressure you set up and then leave running. Cast it at the start of an exchange rather than as a finisher, so the full duration has something to land on.