I found some information on dosage guidelines. Now these come that 2011 study,
Cannabidiol and Other Cannabinoids Reduce Microglial Activation
In Vitro and In Vivo: Relevance to Alzheimer’s Disease, and they are using rats so it isn't a perfect number.
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20mg/kg of CBD, daily for a week, then 3x per week for two weeks (after which, they were "sacrificed").
But consider that dose...20mg/kg
If plant matter is 5%CBD, and someone weighs 75kg, that equates to 1.5g of CBD per dose or 30g of dry material !!!!
You aren't going to get that from smoking; it needs to be concentrates.
I guess that's why the Simpson Oil users work up to eating a gram of oil (or more) a day.
Anyway, in the study the effect of CBD (and these other agonists, which can essentially be considered THC, for the most part) is pronounced, but it is not a cure by any means. However, they are inducing severe conditions in these rats, so perhaps it is a control that pushes the time-frame out so far that other problems of aging are more likely to be expressed in lieu of the Alzheimer's. This is just speculation on my part, though.