how long does someone with dementia and seizures live?

It depends. If a person already has epilepsy or a seizure disorder, that is independent of or precedes the dementia, than the average length of time for the dementia, If we are talking about Alzheimer's dementia then may be three to five years up to fifteen years from the diagnosis, regardless of the seizure disorder if it is controlled. 

On the other hand if a person has had Alzheimer's for a long time and progressed through every stage, and not died from a secondary cause, and therefore developed seizures at the very end stages of Alzheimer's, (as a final progression of the disease) it may be days or weeks or sometimes months till death.