If you are using a light pole, then 45 ft (actual length, not reach) should do the job. I was working on a commercial job on Monday (using a Super-Lite2 45 ft) cleaning 5th storey windows standing back over a row of Mercedes cars, so I was standing about 15-20 ft away from the building.
If you are using using a more archaic (heavier & bendier) pole, then you may need one that's 50 ft (advertised reach, not actual length).