It doesn’t quite work like that I’m afraid. Different species have different ‘perceptions’ of time. Generally, for smaller organisms with a higher metabolic rate time (perceived by them) passes more ‘slowly’. Imagine living life in slow motion as it were. This means that the fruit flies in your example likely perceive much smaller units of time and thus, although we can never know what a fruit fly actually thinks, those 40-50-day lifespans are to the fly what a whole life of 70-80 years or more would be to your average human.