Just to make sure I have this right...
The Consult was subverted by the Dunyain between the series, right? Sometime after TTT, before TGO? Just want to make sure there is no chance that Moengus the elder heard about them from the skin spies he interrogated.
That sounds correct. THere might be a more precise date somewhere, but just on the face of it: at the end of TWP the Consult was looking for the Dunyain. TTT was not a long book, and Moenghus had been torturing the skin spy(ies) in the pit for years, learning how to get information from them. So even if they did find Ishual and start hunting Dunyainm the captured skin spies wouldnt have known about it. He would have had to have found a skin spy that was aware of the happenings up north but was, for some reason, located in Shimeh, then captured it and somehow learned that information in a short time before Kellhus showed up at his door.
So logically it seems to me extremely unlikely that he could have known about the demise of Ishual from skin spies, regardless of the actual date the assault began.
edit:
oops, as H mentions, then you add 4+ years for the whole invasion, plus time to get back to Golgotterath, then I'd assume at least a couple years for subverting the Consult entirely. You're looking at several years after TTT at the earliest.