Stir-frying: how much of the oil evaporates?
The boiling point of most cooking oils is much higher than their smoke points.  The boiling point of olive oil, for example, is around 300°C (572°F), which is hotter than the temperature of a pan on a typical residential range/cooktop.  With that said, alcohols and esters which make up the flavor and fragrance of the oil will have lower boiling points and will therefore evaporate.  That should not significantly alter the nutritional content of the oil.  Furthermore, much of the perceived loss of oil is likely due to a combination of absorption of the oil into the items being fried, and also due to splatter.  The latter cannot be easily quantified due to its connection with the cooking vessel and the technique of the cook.