Any prudent person should realize that the help won't come right away, and one needs to be self-sustaining for a good bit more than the traditional 72 hours.
I think that this hits the nail on the head. The three days/72 hours is a rock bottom "best" case
minimum. What is a realistic period? I don't know, and any guess I make would depend on so many local and personal family variables that it would probably be meaningless to anyone else.
That being said, IMO, a three week preparation of food, water and meds does not seem impossible, nor totally unreasonable or extravagantly expensive. Going to 3 months would be harder, especially in things like water, but not totally impossible given the right conditions. 3 or longer years on your own? Doubtful without a lot of help, or reverting to basically a pre-Columbus native American level of sustainable technology.