A record length for a performance by a single band The Grateful Dead's set at the Winterland Ballroom on New Year's of 1979 lasted six hours. they played 36 songs.
Although they never had an intended time frame for a show, According to Phil Lesh's Autobiography, Searching for the Sound, Their intention was for their shows to be at least the length of, or longer than, the average Acid Trip. Acid being an integral part of the Dead scene for many, the band resolved to play long enough for their fans to dose, trip, and come down so they may get home safely.
You mean "was"not "is", I hope.
They varied from shorter studio and festival appearances up to six hours but the vast majority were 3 to 5 hours including the intermission (as can be discerned from just listening to one of the many concerts that have been released).