We estimate the seismic rupture durations from global large earthquakes (moment magnitude ≥ 7.0) by characterizing changes in the radiated P-wave energy and by introducing the time-averaged cumulative energy rate (TACER), which approximates rupture duration based on the peak first local maximum of an earthquake's high-frequency energy measured at teleseismic broadband seismometers. TACER is particularly useful for real-time evaluations, including the identification of slow-rupturing tsunami earthquakes. In cases of long unilateral earthquake rupture and good azimuthal station distribution, the per-station behavior of TACER may identify the approximate rupture direction, rupture velocity, and length due to directivity effects. We retrospectively analyze 93 earthquakes between 2000 and 2009, and analyze another 65 earthquakes using real-time observations between January 2009 and December 2012. Real-time and retrospective results are comparable and similar to the duration expected from other studies where duration grows as the cubed-root of seismic moment.