Well, it depends how strong you want the motor to be. The longer the wire, the weaker it will be because there will be more resistance in the wire, therefore the total circuit will draw less current and therefore have less power.
In short though, for your situation, the motor is drawing so little current that the length of the wire really does not matter unless you are talking about ridiculously large lengths.
Now, if you are interested, here's all the exciting math behind it
To calculate the approximate resistance of the wire you can use R=(pL/A) where L is the length in meters, A is the cross-sectional area of the wire in meters and p is a constant (if it is copper wire the constant is (1.72x10^-8))
From the specs you have given, the motor draws 16.66666667 mA (I=(V/R)) of current at .0833333333 Watts (P=IV) with no other resistances.
So if you calculate the resistance of 10 ft of let's say 18 gauge copper wire, it will be about .00005 ohms, and then you double it because you have two 10 ft wires running to the motor. So the resistance of both wires will be .0001 ohms.
So with the wires the motor will have .0833333056 Watts of power. With ten feet of wire you will have a difference in power of about .00000003 Watts which is basically nothing.