Dependimg on he size of the floors, but a 70' pole should be fine.
You need to clean at this height differently than you do at say 45' instead of being able to move from one window to the other sideways, you will need to clean them like an esculator, you start at the top floor then you go down the building until you reach 45' then you move sideways and send then pole back up and start again until you have cleaned the building, you then can clean the windows that are below 45' sideways.
Do not try to go around the corner of the building with the pole extended, the wind will rip the pole from you hand.
At this height I would use a modular pole instead of a telecopic one, I also have a golf trolley with a bag that I can deposit and retrieve the poles instead of leaving them on the ground.
Also don't get to close to the building because the wind can get hold of the brushhead and send your pole flying, instead stand futher away from the building and use the angle to get a better grip on the window and building.