Sorry, tell your coach the question needs work. The only way to answer is to assume everything else is fixed and then make a whopping bunch of assumptions. And then its messy because its a messy, non-linear system that also depends on available ceiling height. The theoretical answer depends on math WAY beyond MS, and there is no good rule of thumb.Okay, my coach gave us some homework, and I need to know how rotor pitch affects flight time. <SNIP>
Here's a way to answer the question. Build three copters with different pitches, but same weight, same blade area, number of blades, blade shape.
Now, pick the middle pitch, and match a motor to it to make it fly reasonably good in the space you have. Once you have a good motor size for that pitch, fly the other two motors with the same motor size & winds. What's the answer?
Next, check that result by matching a motor to the lower pitch. Probably different size and winds than before. Use that motor, size and winds on the other two copters and see what happens.
Match a motor to the high pitch copter. Try on the two other copters. Result?
Was the result consistent, wherever you started?
If you don't understand what I'm saying when I say match the motor to the pitch, look back up this string.
Hope that helps,