Construction on WWoHP took about 2.5 years. Sure, they had existing coasters, kitchen for Three Brommsticks, and I think the restrooms too, but they built the whole of Hogsmeade and the whole incredible FJ attraction within that time. Amity also has existing restrooms, and we don't know that they'll be filling in the Jaws waterways. If the rumors for a Gringotts coaster turn out to be true, they could even utilize the different levels in the Jaws ride. They just need to build it up and create a rock texture all around, and voila! You got the Gringotts' caves. I guess it's just a waiting game now...