I made a terrain with "Rock" as material, (the very first material in "Displacement Materials") and some transparent water over it, The terrain is 130K polygons, the waterplane only 1100. After 1 hour the 4000x2500 render was still on "0" and it said 678 h, 14 min left.

I have a fairly new Quad computer with 8GB memory and GeForce GTX 465 videocard. I was complelty baffled. I hadn't even get started yet!

As you know you can't print with less than 3000,4000px with any quality.