Well, you can not achieve a real plane wave. But you can create an approximation to it. What you need is an electromagnetic wave whoose amplitude and phase does not vary over a certain area. So the distance required to obtain such an approximation does depend on the size of the antenna or the aperture of the antenna.
Suppose i have a 1m monopole antenna on top of a car, transmitter frequency is 140MHz and the eviroment is reflection free, what should be the distance from transmitter to receiver?