There really is not a rule of thumb because it depends on how much other light is in the room. In our kitchens, we like to space the halogen or LED potlights about 4 feet apart. If there are potlights in the kitchen along with the pendants, then 4 to 5 feet is good for the pendants. An experienced, qualified electrician can give you good advice and you can also take your plan to a good lighting store for advice.