Centuries and millennia? Those weren't nexuses, but large portions of Sylvania's waystones were connected to Mordheim before it was destroyed. They then underwent centuries of degradation, before a good chunk of them were destroyed by vampires and necromancers under Mannfred and Konrad. Sure, those are waystones. But waystones are man-high rocks. These are mountains we're talking about.
We have an idea of how long it takes for the reactivation of waystones after periods of dormancy takes for it to be noticeable. That time period is decades. I don't see why the Dwarfs of the Golden Age would have built their waystones to be so vulnerable. If nothing else the fact that they survived the Time of Woes proves they didn't make them delicate.
Oh right I was thinking of the Waystone Clog, but then than would have been much more slapdash than just turning it off... then again that is also a Karaz Ankor Waystone whereas the one in the quote is not. I think this might be worth asking the GM @Boney assuming one of the Karaz Ankor stones was blocked and the flow had nowhere to go how long does she expect would take for noticeable Dhar accumulation to happen? Days? Weeks? Or more?