How long after eating is my stomach considered empty?

Generally, the idea is that you take your medication shortly before meals, with a glass of water. You don't usually have to be certain that there is literally nothing at all in your stomach; it's just that if you have eaten a meal in the last couple of hours, say, it will interfere with how well the medication is absorbed.

But this does depend on what medication you're taking. A few need special precautions: in that case, seek advice from your prescribing doctor or pharmacist. The patient information leaflet should tell you what you're dealing with.