Standards for this are hard to come by and it is ultimately up to the hanger. In hanging shows for the school gallery where I was director for over 20 years, we would place the center of the image (not the frame) 63 inches from the floor. Some felt that this was too low, others felt it to be too high. "Eye level" is not a constant. If you are 6' 4" tall, you are going to hang the picture much higher with that criteria than if you are a 5' 6". Commercial displays of art (galleries, shops) have to compromise. 63 inches was our compromise. If you hang at home, hang it where you're comfortable and ignore convention.