I think you're confusing "generation" and "life span." Your model may be bad in other ways as well.
Generations would be even shorter. Life span for a fruit fly from egg to death is one week. Generations would be even shorter. I was being generous.
I don't care how it's set up. Set it up anyway you like. Find a shorter lived creature like a gastrotrich, some of which which has a 3 day life span.
The point being that surely someone should be able to make a fruit fly change evolve into something other than a fruit fly. Yet it hasn't happened. Why?