How much sugar is too much?
There isn't a precise answer that fits everybody. The WHO has issued a guideline about added sugar, meaning sugar that doesn't naturally occur in fruits and vegetables. They suggest limiting it to a daily average of 5% of your caloric intake. For most adults that translates to about 25 grams per day. That's actually a fair amount of sugar, and it's pretty easy to limit yourself to that, especially if you eschew sweet drinks (fruit juice and soda pop) most of the time.
Note that I said daily average. Go ahead and have that milkshake or pie every now and then. Just moderate your overall sugar intake.