The viewing distance of an image is based on two factors; first is the diagonal image size and second are the pixels per inch required at that distance to give a sharp image.
Firstly the rough rule of thumb is that the viewing distance should be 1.5 to 2 times the diagonal length. This will give you an optimal viewing distance for the overall printed size based on the human eye's ideal viewing angle. You have to understand, however, that for a landscape this may not be optimal as you may actually want the viewer to pan around the image, and you may want the size of features within the image to be the basis of this calculation. This is an artistic decision though, based on the composition of your image.
Secondly for the image to look good at the distance you choose, there need to be sufficient pixels per inch (ppi) to fool the eye into seeing a smooth image that isn't pixelated. The minimum ppi needed for a print with acceptable quality is calculated by dividing the value 3438 by the viewing distance. Anything above this ppi will look good at the distance chosen.
So: minimum ppi = 3438/Viewing Distance
With viewing distance in inches, and where 3438 is a constant for human vision, which was derived as follows:
1/ppi = 2 x Viewing Distance x tan(0.000290888/2)
1/ppi = Viewing Distance x tan(0.000290888)
ppi = 3438/Viewing Distance
where 0.000290888 radians (1 arc minute) is known as the 'visual acuity angle' and represents how much resolution a human can see.