We used the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) to identify and quantify the duration of relativistic, >1 MeV, electron microbursts. A typical relativistic microburst has a ~100 millisecond (ms) duration, and the interquartile range of the duration distribution is 70-140 ms. We investigated trends in the microburst duration as a function of geomagnetic activity, L-shell, and magnetic local time (MLT). The clearest trend is in MLT: the median microburst duration doubles from 80 milliseconds at midnight to 160 milliseconds noon MLT. This trend is similar to the whistler mode chorus rising tone element duration trend, suggesting a possible relationship.