How many hours of test flight does a general aviation aircraft go through before delivery?
All manufacturers have a quality system in place, and they also have a production baseline (which is a result of certification). so, as long as the quality system is utilized, there is technically no need to fly each manufactured airplane. Type certification validates the production baseline (and that each manufactured plane is within the tolerances acceptable for the certification authority or requirements). 

However, depending on company manufacturing know how (or lack of) there might be several ground tests (structural, engine, systems checkout) before delivery.

On the other hand, for many military aircraft there's a factory checkout flight, to check and confirm the performance of each frame and propulsion system. This is more useful for risk reduction (to avoid customer complaints and to check the complex system in the air).