SMN target swap - how many GCDs to break even?

[SunnyHirose]

This I'm confused about. You get the full potency of a dot tick whether you cast it 2.5s or 0.5s before the server tick happens. There is no prorating.

In theorycrafting a DoT with some sort of time cutoff (e.g. clipping the DoT or considering a burst period), it is often a useful abstraction to consider its damage probabilistically, as if the timing of your cast and subsequent cutoff is so unrelated to the tick timing that you have an equal chance of landing it anywhere inside the 3s window.