I think the longest I've waited for a new part in a WIP and actually got one was well over two years or so for a TS WIP. Which was in the end even more crushing, because after the long hiatus (during which I had mostly given up on it) then there were posted three or four chapters in more or less regular intervals (like every couple of weeks), only for the WIP to be abandoned near the end after all, and pulled from the net. Actually I think I've waited even longer for another TS WIP, but there the update was only a little part. IIRC that WIP had been already abandoned for some time when I joined the fandom in early 1998 and I've seen an update in late 2000.