What will the table be used for? Will kids be standing making models of volcanoes? In that case hang it higher. Are people always seated eating at the table? Then you can hang it lower.

I think this is personal preference, and, as Tribble.... says, it depends on use. You could also consider looping the cable decoratively, so you could potentially adjust the height later if you wanted to. I've attached an image of a light we installed as an example.

Like the previous comments it depends on use and the design of the light fitting itself. Also if the light has a narrow beam it may need to sit higher to throw a wider beam to light up the area you need lit up. Generally you would put it above eye level so that it doesn't obscure your view when standing. More often than not it is just a matter of where you think it looks best, most pendant lights have good adjustment so before its installed have the electrician hold it up for you at different heights and see what looks best.

We placed ours 1 metre from the table , it creates an intimate cosy feel without being in the way.