53 minutes, 5 seconds, assuming you want to count the credits and opening.
The ricin will be used one of the following ways: Walt uses it on Jesse, Walt uses it on himself, Walt uses it on Lydia or Todd.
Lets see who has a reason to kill Walt,
Marie, Skyler, and Flynn: All three of them could want to eliminate Walt for Hank.
Jesse: His life is absolutely shit right now, he has the most reasons to eliminate Walt.
Saul's team: Huell was told, by Hank, that Saul was killed by Walt. I doubt Saul will be involved in Walt's death.
DEA: Hank and Gomez vanish, assumed dead, everyone knows Walt is Heisenberg, Walt is prime suspect.
Jack and the Nazis: Loose ends. Get rid of Walt so there is no one left that knew the truth behind Hank's death. Jesse won't try to run anymore, so they have him out of the picture already.