How long do i have to wait after eating before i can take a supplement or medication that is suppose to be taken on an "empty stomach" in order for it to work effectively? is metabolism also factor?

Sometime after 2Hrs.: It can take stomach 2-4 hours to empty. But most of the food has left the stomach in 2 hours unless it was a fat rich meal which takes longer to move out of the stomach(3-6 hours). So use your best judgment.

Gastric emptying: On average, it will take 3 hours for your stomach to empty after a meal. Best to take right before a meal.