I actually believe there are three more volumes to be published--the other one being on the Progressive era through the 1920s. They just published the volume on Reconstruction and the Gilded Age a month or two ago, and the shortest time in between publications of a new volume has been two years. Since a volume was just recently published, I would expect that two years seems like a good minimum estimate before we see the newest volume. I would expect it to be a decade or so before the series is completed going off of the current publication schedule.

To get anything more exact would probably require a bit of inside info