As Slow As Possible is another composition by John Cage. As written, a performance lasts 20 minutes, however there is a performance in process right now which began in 2001 with 1 1/2 years of silence and which won't be completed until 2640. The work was originally written for solo piano and later adapted by Cage for organ. Sheet music is available here. You can hear a 30 second excerpt of the solo piano version here.
A part of the long version of this piece can be seen performed here:
and another part here:
I can't find the 20-minute version for piano online anywhere.