It takes around 10 days to day from dehydration and when you're dying from dehydration it's usually a pretty easy way to die and not really painful.
If you stop eating anything along with drinking then dying from dehydration and starvation can be as early as 2 days.
If you're dehydrated you need to get hydrated pretty soon to avoid health problems and if you're severely dehydrated then you may even need an IV at the hospital.