I'd say you can eat pretty soon after taking the supplement (maybe wait five minutes or so!).
The reason some supps. and medication are supposed to be taken on an empty stomach is to avoid having them become digested in the stomach (most likely because stomach acids/enzymes will destroy it before it can reach the intestines). So when your stomach is empty, it is also effectively "open" to the small intestine (when you eat food, the digestion process kicks into gear and the stomach "closes" up to allow for digestion).
So when you take your pills on empty stomach, they move quickly though the stomach and into the small intestine (meaning it's safe to eat quite soon after taking them).