It was an ongoing task, since Rand entered Arad Doman and tried to make order there. Graendal had to observe what Rand does and then try to counter it.

As said, there is no hint that Graendal was actually concentrated on the optional task of giving Rand pain of heart just for some unknown reward and with the risk that this might incur Rand's wrath to her (and if Graendal isn't an idiot, she should realize that this "reward" is hardly worth much, especially if she still wants to become Nae'blis herself).

In the same way one could argue that Graendal was concentrated on searching the seals or Mat and Perrin.