Paintings should be hung at eye level. The way to accomplish this is to measure the length of the painting and divide that by two. Add 60 inches (152 cm) to this. Using that number, make a small mark on the wall at that height.
For example, if your painting is 36 inches (91 cm), then half of it would be 18 inches (45.7 cm). So the hanger for the painting should be at roughly 78 inches (198 cm).