This article is from the Epilepsy FAQ, by Andrew Patrick (firstname.lastname@example.org) with numerous contributions by others.
Depending on the type of seizure, they can last anywhere from a few
seconds to several minutes. In rare cases, seizures can last many
hours. For example, a tonic-clonic seizure typically lasts 1-7 minutes.
Absence seizures may only last a few seconds, while complex partial
seizures range from 30 seconds to 2-3 minutes. "Status Epilepticus"
refers to prolonged seizures that can last for many hours, and this can
be a serious medical condition. In most cases, however, seizures are
fairly short and little first aid is required.