"My dad has dementia. We have CBD oil derived from hemp. How much and how often should he take it?"
Here is an article on the subject https://www.tokeofthetown.com/2013/02/cbd_reverses_dementia_in_australian_study.php/
there is no standard dosing for cannabis. Make sure that you have a product that has therapeutic levels of CBD. Does your label on your hemp product say how much CBD is in the bottle? CBD from cannabis strains usually have much higher levels of CBD than derived from hemp.