Our perception of what moderation is is shaped by our culture. Today, people eat an average of almost 150lbs of sugar per year, whereas three hundred years ago the average consumption would have been nearer 5lbs per year. Modern-day levels are toxic to the body, which is one reason why there are so many chronic metabolic disorders.
Now you may think it's okay to indulge, as long as you don't do it too frequently?
Let's look at it as a stressor to the body, and use exercise as a comparison. If you do resistance exercise sufficiently (but not too) intense, your body will grow stronger in about 24.2 hours (circadian rhythm). If you don't train within 4 and 1/2 days you will lose any strength gained. If you don't train within 9 days, you will be weaker than you were last session. That demonstrates how long the effects can last.
Another exercise comparison is a marathon; when electron microscopes were aimed at the legs of marathon runners, they found evidence they had not fully recovered months later.
So how long is eating too frequently for a toxic-dose of sugar?