How can I find the optimum chord wing length for this wing? This Aviation SE forum looks the perfect place to ask this question: people here usually make maths the root of most solutions and I think this may solve my aerodynamic optimization problem!

I am having a very hard time optimizing the wing size of my rc plane. I already went to the excelent RC GROUPS community but, despite the generous help of many people, this subject got a little more confusing and personal opinions (practical experience) are conflicting.

My question is this: I already have an rc plane (with tail, fuselage...) and I want to optimize the wing size in order to get efficiency and the longer flight time. I already built dozens of rc planes but this time I want to make this one the most efficient as possible.

IMHO to get longer flight time I need to consume the least amount of energy from my lipo battery as possible - so I need to keep my brushless motor running in the slowest RPM possible, consuming less Amps. Having said that, my total plane weight (not considering the wing, of course) is around 300g (summed weight of ESC, servos, 1000kv motor, 11 inch prop, fuselage, tail, receiver and battery). The weight is fixed, 300g. My wingspan is also fixed, 90cm. I cant have a wing longer than that. The wing shape is rectangular (I know elipses would be the better shape but let's fix the wing shape to simplify this discussion).

If everthing is "fixed" so what is the variable? Well, I want to find the optimum chord wing length for this plane which is gonna bring me the highest flight duration.

I am using the CLARK Y airfoil but you can assume any other one you want. I wouldnt like to discuss airfoil, I would like to discuss the size of the chord and how to optimize it.

My experience and lots of study into this subject says that high aspect ratio wing are usually more efficient than low AR. My wingspan is fixed and all other variables are fixed (to make this discussion more specific) except chord. 

So my dear friends, I am struggling a lot to find the optimium chord to get the best flight duration. It looks easy but I am almost 2 months trying to get this answer from many different sources and I still cant find a good solution.

PS: I know I could optimize many of my other fixed parameters, but to avoid having this question marked as too broad/global/generic, I really would like to only optimize the wing chord.

PS (2): the plane that I want to optimize the chord is in the image below. The wing that I show on the picture already provides good flight time (around 22 minutes) but I would like to optimize it in order to get better flight time.
You want to fly at the polar point of the lowest energy consumption. If you describe the drag with the usual parabolic equation, you can use the optimum point derived in this answer. It is
$$3\cdot c_{D0} = \frac{c^2_L}{\pi\cdot AR\cdot\epsilon}$$

Induced drag is three times the parasitic or zero-lift drag when you fly at the polar point for minimum energy loss. But the minimum induced drag at a certain speed is also fixed once wingspan and mass are given - you can only mess up by creating a non-elliptic circulation over span, but never get better than this limit.

Now get the zero-lift drag down as much as you can (pick a thin, highly cambered airfoil which produces a high lift coefficient at the given Reynolds number, definitely no Clark Y) and size chord such that this airfoil will work at the optimum lift coefficient.

Note that this will be an iterative process. Picking a chord will influence your aspect ratio and zero-lift drag, so the optimum lift coefficient will be depend on the chosen chord.