Is it the frequency or the length which is considered more serious with seizures?

Both important: If a seizure is continuous and lasting longer than 5 minutes or recurs without regaining consciousness between seizures for greater than 5 minutes then you have status epilepticus which is life threatening. If you have frequent seizures (somewhat arbitrary as to the number) then there is a higher risk for further complications as the brain "learns" how to have a seizure- a process called kindling.