Although you'll never know for sure unless you're the person who is dying from dehydration studies have shown that once the initial thirst subsides dying from dehydration is usually a non painful and peaceful way of dying.
Some hospice patients decide to not drink any fluids so they can die sooner but it can take up to 10 days to die from dehydration.
Most people will last up to 10 days without anything to drink so it won't be a quick way to die but it will take awhile.