How much head do you throw away when distilling?

I'm curious, when it comes to the process of distilling, how much of the initial material do you actually discard or "throw away"? Is there a specific ratio or percentage that is commonly used, or does it vary depending on the type of liquid being distilled and the desired outcome? Additionally, how does the amount of discarded material impact the quality and purity of the final product? Understanding this aspect of distillation can help inform my decision-making process when it comes to choosing products that have undergone this process.