Routine EEGs record about 30 minutes of brain waves, typically with a flashing strobe light and deep breathing test. This is essentially a brief screening exam to determine if there is epileptic activity. However, 30 minutes is often not long enough to catch abnormalities.
Long EEGs can range from 1-3 days or sometimes longer, depending what type of activity is seen. Long EEGs are used to capture any symptoms a patient is experiencing, to determine if it is related to seizure activity or not. It is also used to screen for "silent" seizures - also known as subclinical seizures. Often, patients are having tiny seizures that they aren't aware of, and this can contribute to major problems with memory/focus/attention. Depending on the history of the patient, the epileptologist will determine if a long study is needed, and if overnight/sleep is required for diagnosis.
Please note that a normal EEG does NOT mean you do not still need an MRI - EEGs evaluate the electrical activity of the brain whereas MRIs evaluate the structure.