That is because you can vomit up food and possibly aspirate it. How fast your stomach alone empties is based on several factors from the contents of the last meal you ate to your overall metabolism. Generally, in a moderate calorie (non high fat) meal, you could probably expect your stamach to have emptied within 2-3 hours or less.
The real issue is, why does this matter? Most supplements or medications that indicate being taken on an empty stomach don't expect anyone to wait half a day before eating again or to take an ultrasound or x-ray just to see if their stomach is completely empty. For most of us into bodybuilding who eat frequently all day long, stomach emptying would take place sooner.
That means, if you are looking for a general recommendation, wait two hours after eating assuming your last meal wasn't two large pepperoni pizzas with extra cheese.