They could not have known the exact time the laser would have been used to it's possible the TFR to up a significant period of time.
The lasers were only used during 1 or 2 songs and yes, their shows did NOT vary in length. The link I posted explains it and also informs you of how powerful the Copper color laser was.....
But yes, during those one or two songs, no Air Traffic in the vicinity (I do not know the exact figure) because of how powerful they were.
Edit: "
With a similar divergence to argon lasers, a Copper Vapour laser beam keeps it's size for long distances. One consequence of this is that when the tour is playing at open air venues and is projecting unterminated beams, it has to work with local Air Traffic Control to declare an exclusion zone around the stadium." Mind you almost 1/2-3/4's of the venues............were open air.
They used these exact lasers to remove the "twinkling of stars" in pictures, because of the disruptive "atmosphere" they caused. As far as I know, Pink Floyd were the only ones to ever use this "potent" of a laser.