Time and duration are at the heart of many of Ragnar Kjartansson’s performance works. In 2011 he staged a 12-hour, looped rendition of a single Mozart aria; he pushed the pop-song format to absurd lengths with his 2012 installation The Visitors, stretching one tune to 64 minutes. Time is of the essence in this case, because “A Lot of Sorrow,” his show at Luhring Augustine, finishes its three-month run on Dec. 21. The six-hour-long video shows the rock band The National in performance at the VW Dome in the courtyard of MoMA PS1; the band, dressed in black suits, performs a six-hour rendition of their song Sorrow, normally just three minutes. “I don’t want to get over you,” sings the jilted lover in the song. In his 2007 film installation God, the artist himself sings that “sorrow conquers happiness.” But somehow the band’s protracted expression of sadness brings true joy.