As much as you can get. But Walmart has that 10 dollar prescription plan where they'll sell you up to 90 days in advance. If you have essential prescriptions, you might want to consider going out of pocket for the 90. The list is HUGE of the drugs you can get, generics only - but hey, you gonna complain when you're dying?
If you have insurance, check with them. Mine has a mail order RX service and they send them 90 days at a time. I got my Rx's refilled by my doctor (retail), then turned them in for mail order and recieved my order about 10 days later. Now I always have good supply of my meds.
The 30 day limit thing has more to do with insurance providers, and less to do with the medicine itself. The prescriptions from Walmart can be up to a 90 day supply, and as long as you pay cash (10 bucks) it's irrelevant. My "dodge" from my physician is that I would be backcountry for extended periods and have had "issues" with things like guardia and a compromised immune system - so the prescriptions themselves are prophylactic in nature and the doctor doesn't get in trouble. LEAVE THE LABEL ON the prescription, possesison of prescription meds that are not your own is illegal - you don't want unlabeled stuff.
You will most likely only have to deal with localized emergencies - such as hurricanes. In this case a 2 to 4 hour drive will get you out of the zone and you can get whatever you want. Gas, Food, Drugs, etc.
In the case of a national disaster - nuclear war, germ warfare, etc, then you just pick up one of those guns you have stockpiled and go get you some. Where from? Who knows.
Now the problem will be that before you need meds you will need food. Grocery stores will be out in 2 days tops, and I do mean tops. Since this is about meds I will not go further than this on food. Same with generalized drugs.
If you need the meds in question to live, then you need to look into ways to make them or something close enough (and it may be crude) to keep you alive. You can stockpile food - but stockpiling drugs is almost impossible with the laws we have. Look for alternatives to the medicine that can be found in nature and start growing that plant only found in southern Brazil...
If it is really specialized then read up on Darwin or get a Chemistry degree. Talk to a chemist, get to know your neighbors and their abilities.
I was going to start a thread on this, but took the time to search this topic out and read. I have HBP and high cholesterol. I want to ask my Dr. about an Rx for this and then just buy them without insurance butting their nose in my business. I am not against insurance companies but the whole situation has turned out to be a big mess and it IS out of control at this point.
If you love the post office your gonna love gov't run health care. They couldn't wipe their own butt if they tried. Everything and i mean everything the touch is a screwed up mess. It's a wonder that the Military can function as it does.