Although I can't see the udder as well as I would like, I would say within a week/10 days tops. Beef animals are different than dairy in they won't fill as hard or as tight as most dairy animals before calving... the teats look pretty well filled/ firm but not tight. Most beef calves cannot deal with the hard tight udders on a fresh cow and for the most part I think that is nature's design to not have them quite as tight.
If you are seeing a good amount of discharge, and she is pretty loose... that is a good sign in my book to watch a HEIFER close, and a calf is probably within a week or so.
When the vet diagnosed her due date, did you have an approx time frame of when she was exposed? Also, how far bred was she... they are much more accurate of how far along they are if checked before 90-100 days... after 3 +/- months pregnant, the calf is more over the pelvic bones, and they are very hard to be very accurate if 5 + months bred...
Another thing... was she "wider" and now actually seems slimmer on her sides? That often means the calf has turned and is more position to calve....she will actually look "less pregnant" just before she has it.....