Assuming that Chronicle image is a somewhat reliable source (and its been questioned lately with the whole Transbay issue), the reason they seem to have slowed down for a bit, could be because they were installing earthquake retrofiting beams and such near the 1/3 point of the tower (where the first yellow box has been outlined). After getting through here (if they havent already done so) they should resume thier pace until they near the top (or the second yellow box). I would assume they will top off around June, possibly July. They should finish all the other mechanisims, glass, wires and pipes, by around September perhaps, with the first people moving in around October-November.