Not very long, because rain blows in and/or water drips off the open garage door even if I don't drive it.
Obviously not what you meant, though . . .
Clearly the back gets dirty while driving due to turbulence/vortices. So, anything that cleaned up the aerodynamics should not only improve range but also keep the hatch cleaner. Lots of possibilities - splitter plate, etc. - but the real question (IMO) is what might do the trick w/o being obtrusive. Two ideas come to mind:
1) a different wing/spoiler, e.g., the one of the JCW or one of the various aftermarket options; and/or
2) better management of the airflow under the car, e.g., sealing off under the bumper or a different rear diffuser.
Anyone with any relevant experience that they would like to share?