The concert began in 2001 at the former St. Burchard church in Halberstadt, Germany, and is scheduled to conclude in 2640, for a duration of 639 years. Though Cage did not specify a duration for his piece, the length was chosen because the start date for the performance is 639 years after Nicolaus Faber built the first church organ. Other performances of the piece have lasted between 29 and 70 minutes.
The Halberstadt performance, by an automated organ, progresses so slowly that visitors have to wait months for a chord change (for one note change, in 2008, more than 1,000 visitors came to listen). They occur on the fifth of the month, but the next one isn't scheduled until July 2012. The organ emits a low meditative hum at all times. You can hear it on the ASLSP Web site.
The performance is so slow that the organ it is being played on was not even complete before the music began. Pipes were added to the organ, which keeps the music steady thanks to a machine called a blower that provides a constant stream of air, in 2008.