Most electroconvulsive therapy (ECT) research indicates that seizure length does not correlate with clinical efficacy. However, it is common in practice for clinicians to undertake measures to prolong seizures if the duration seems to be too short, although there is no universally agreed upon minimum seizure duration for ECT. We felt it would be informative for the ECT field to report mean seizure durations over the course of treatments based on age and sex in a very large cohort to provide norms for reference. We studied 519 patients' courses of ECT and recorded treatment number, sex, and age along with motor and electroencephalogram seizure duration. We found that women have longer seizures, but only at the first treatment session. There is a strong inverse correlation between age and seizure length. The biggest drop in seizure duration along a course of treatments occurs between the first and second treatments; beyond that, seizure duration remains relatively constant.