My two cents worth, for creating an image , expecially for print purposes, have been going to 4096 x 3072 (print is 36 x 24 inches, 90 x 60 cm), rendered at ultra max settings took 24 hrs, 4.5mill polys, lots of shadows of grass, leaves etc. and high res textures on trees, bark and leaves, no obvious grain, as mentioned above, higher quality results higher output (for sure worth it), PC dependent though on time.