To me, it depends on what type of rub you are using.
If it's mostly sugar, then I would pile it on, becasue most of it will melt and run off.
If it's a more savory rub...mostly paprika, then I would tend to use less because it stays around better.
A cup should do it if it's a paprika rub. Maybe 1 1/2 - 2 cups if it's more sugary..