In every healthy diet, with the exception of keto and related diets, a bit more than half of all your calories will be from carbohydrates (carbs). Carbs have a glycemic index (GI). It's a number from 0 to 100, 100 belonging to glucose, of the relative increase in blood glucose two hours after eating a certain carb. If potatoes have a GI of 20, it means that two hours after eating potatoes, they'll increase your blood glucose to 20% of if you ate pure glucose.
A sudden rise in blood glucose, associated with high GI foods, is seen in people whose cells are becoming resistant to insulin. Simple carbs, like sugary food, fruit, most of the sweet stuff you know, have high GI. More complex carbs, beginning from starch to fibers, have lower GI.
That's why fibers are good for you: They take up space in your stomach, preventing you from eating more, and inducing hormonal responses related to satiety that are caused by the stretching of the stomach walls.
(Among other health benefits)
In type 2 diabetes, the type that appears later on in life and is linked in many different ways to eating a lot of sugary and fatty food, your cells have become very resistant to insulin. The body produces insulin, even in greater amounts than before, but this insulin cannot make the cells absorb the glucose from the blood. So all in all, sugary food -> insulin resistance -> diabetes.
But how much? As I said in the beginning, this is a mechanism we propose to explain what we observe. It stands to reason that less consumption of sugary food is less likely to induce insulin resistance. But it's too complex and it's affected by too many variables for a simple easy defined range. You can't say "if you eat 300 to 500 calories of high GI foods, you're safe, and it becomes unsafe more than that". That's just not how biology works.
So apples and oranges can be pretty sweet, and they have high GI to be sure, so why aren't they as reviled as, say, chocolate products?
The answer is three-fold: 1) Sweet products like candies are addictively appealing. A medium-sized apple is around 100 Cal. A medium-sized snack of sweetstuff is roughly 500 Cal. You don't hear anyone having 5 whole apples for a snack, but a couple of biscuits? Sure.
2) They are very nutritious, unlike the so-called "empty calorie" foods. Vitamins, fiber, other good stuff, and no fat, unlike, say, chocolate bars.
3) You already knew this, they consist of a lot of fructose which is half as sweet as glucose, and is "healthier".
However, too much fructose isn't good for your body either. Excess fructose will be delivered to the liver to enter the TCA cycle, instead of the normal fuel, which is glucose and especially fats, thus slowing fat metabolism. IOW, you can get fat by consuming too much fruit.
So if you don't have a cutoff point for consuming sugary food, how much soda should you drink? My advice is to keep it simple: Make sure your daily meals are intact, and eat a diverse range of foods to ensure you get the nutrients your body needs. If you count the calories for your meals and they add up to less than 1800, filling the gaps by indugling yourself in some sweets is not not okay.