Standard height for pendant lighting hung over a kitchen island is 30 to 32 inches above the countertop. Use this height as your starting point, and adjust up or down depending on several factors. It may be best to have someone else hold the pendant light at 30 to 32 inches over the kitchen island while you step back and analyze it.
When the pendant is hung over the kitchen island, you should be able to see at least 5 inches of the chain. If you can't see at least that much, adjust your pendant lower, closer to the kitchen countertop.
The tallest person in the house should stand below the pendant to ensure that it hangs at least two inches taller than his head. The pendant should be hung at a height that gives that person a clear visual path across the island. If it's hung too low -- lower than 2 inches above his head -- the light fixture will obstruct the view and become a nuisance.
With the light on, place a plate on the kitchen island. If the light is producing "hot spots," or intense sections of bright light on the counter, it's hung too low and should be raised. Ideally, the light will cast an even glow on the workspace, and adjusting the height of the pendant will help achieve that glow.
Ultimately, what's most important is that the light looks right to you. The standard 30- to 32-inch height is a guideline to use a starting point, but if the light seems to high or too low to you, you're probably right. Sometimes in homes with high ceilings, for example, it makes the most sense for the light to be hung higher, closer to the ceiling, to fit the scale of the space.