The interictal electroencephalogram (EEG) has an important role in the classification and treatment of epilepsy. In busy EEG laboratories, valuable resources are used in order to comply with current recommendations regarding the length of EEG recordings. Our aim was to examine the time to first interictal epileptiform discharge (IED) in standard and sleep-deprived EEGs. Standard and sleep-deprived EEG recordings with IEDs were retrospectively reviewed during a 2-year period. Bedside EEGs and long-term video-EEGs were excluded. IED latency according to EEG type, age group and inpatient/outpatient status was analyzed with the Kaplan-Meier estimator. The study group included 684 patients, 372 (54%) males, aged 0.2-89 years. Standard (n = 316) and sleep-deprived (n = 368) EEGs were performed in 245 inpatients and 439 outpatients. The EEG was requested in 96% of the inpatients following a seizure. Most IEDs were recorded whilst the patients were awake (43%) or drowsy (34%). Ninety percent of the IEDs were recorded within 18.5 min, earlier in standard (14.6 vs. 21.3 min) (P = 0.024) EEGs and in inpatients (14 vs. 21.3 min) (P = 0.002). IED latency was unaffected by age. Electroencephalogram type and admission status may be used for individual determination of the duration of EEG recording. Reducing the duration of standard and sleep-deprived EEGs may be considered especially in inpatients.