Glad you asked!
There is actually a range where you should hang your art. If you are looking for a quick answer, it's about 57-60 inches from the floor and the center of your art.
The biggest mistake I see people make is hanging art too high, but the sweet spot is at eye level. What if you have 20-foot ceilings? The rule stays the same, and the magic number is 60". Of course, there are exceptions to this rule. For example, if you are a family of really tall people, your eye level will be a couple of inches higher.
Gallery wall? The same rule applies when working with several frames, the center of the grouping (rather than the center of any one picture) should hit the 57-inch level on the wall.
Et Voila! You can now start collecting my art and hang it correctly!