Fructose has beneficial effects on metabolism in the amounts found in fruits. Fructose at higher quantities without the vitamins, minerals, enzymes, and fiber found in fruits can cause problems such as non-alcoholic fatty liver disease. But what I just described is basically just soda. Something like 34g of fructose a day has the beneficial effects. So just for the beneficial effects, it seems like one should eat several pieces of fruit a day, say 1-2 at each meal.

Depends on the fruit, but how about 3?