Perhaps you've heard: Tonight at 6:30 p.m., Eastern Time, some of the biggest, fastest, strongest and richest men in America will meet on a field of painted grass in Indianapolis and practice war games. They will wear shoulder pads, and they will be watched by millions. If you would like to be among those millions, the event is streaming here. (Note: That's the only place it's streaming. If you don't want to stream, tune into NBC.)
There's no telling exactly when Madonna will take the stage for the Half Time show, but the earliest it could possibly be is 7:30, Eastern Standard. 8:30 p.m.'s most likely, it almost certainly won't be after 9:00.
UPDATE: Sorry! The livestream skipped the Halftime show completely. I just posted a vid of Madonna's performance, which you can watch here.