Michael Tilson Thomas, successor of the titanic Leonard Bernstein as America’s foremost champion of Gustav Mahler, takes on the last of the master’s completed symphonies. Beginning and ending with oceanic slow movements, the valedictory Ninth can run anywhere from 75 to 90 minutes. It’s not unusual for conductors to slow down with age, but can the line be stretched any further? An MTT Ninth with the San Francisco Symphony, recorded two decades ago, clocked in at 89 minutes. Timekeepers, to your posts. —Matthew Gurewitsch