Anybody with a passing knowledge of aerodynamic principles would question inverted flight. How do the plane's wings create low pressure above the wing in regular flight, and create high pressure in upside down flight. 

Bernoulli’s Principle

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I will not be going into the exact numbers and formulas of flight, but will rather be assessing the qualitative aspects of what makes planes fly. Basically, the shape of the airfoil (jargon for cross section of wing) causes a pressure difference above and below the wing. The under side of the wing has faster airflow, causing higher pressure, while the upper part of the wing has slow moving air, causing low pressure. This pressure difference leads to an upward force called lift. This is how planes fly.

How do planes fly UPSIDE DOWN ?

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The physics we exploit lies in the angle of attack, that is the angle of the airfoil relative to the airflow. For both scenarios, upside down and right side up, the pilot can adjust the angle of attack so as to create a pressure difference and create lift. Another feature for stunt planes is the symmetrical airfoil shape which allow for similar angles of attack, and similar lift patterns. 
Time that can be spent flying upside down all depends upon pilot control, engine, airfoil design etc.

Conclusion

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It is not the direction of the wings that matters but rather the shape of the wing, angle of attack and more essentially the pressure difference formed by the air flow. For those further interested in the quantitative aspects of this pressure difference, this is Bernoulli's principle, constant is derived experimentally: 
P + 1/2 ρ v² + ρ g h = constant
The Declared Values
  • P = static pressure
  • ρ = fluid density
  • v = fluid speed
  • g = acceleration due to gravity
  • h = height