It may take 18 - 24 months for construction, but if construction took place off site and started before GMR closed, then only a few months of downtime would be needed to gut and install everything. That's pure speculation though. Hall of Presidents is taking 6 months because they are likely going to have new visuals and narration added, as well as updates to some current animatronics and maybe even a speech from the new president himself. I'd say a 6 month time frame is pretty good for doing all of that.