The short answer? An absolutely comical amount, perhaps lit by a jet engine. It would also help if you owned a residence in Toontown or were physically a Tex Avery illustration. In a 1988 DEA brief, Judge Francis Young did the math and broke down just how much sticky-icky it would take to off you entirely:

It is estimated that marijuana's [median lethal dose] is around 1:20,000 or 1:40,000. In layman terms this means that in order to induce death a marijuana smoker would have to consume 20,000 to 40,000 times as much marijuana as is contained in one marijuana cigarette. [National Institute on Drug Abuse]-supplied marijuana cigarettes weigh approximately .9 grams. A smoker would theoretically have to consume nearly 1,500 pounds of marijuana within about fifteen minutes to induce a lethal response.

Young goes on to surmise, "In practical terms, marijuana cannot induce a lethal response as a result of drug-related toxicity." But what if you actually wanted to, in completely impractical terms? I imagine that you would have to build a hotbox the size of a ziggurat and ignite your stash via orbital laser.

Via Dangerous Minds. Top song — Scientist's "Your Teeth In My Neck" from Scientist Rids the World of the Evil Curse of the Vampires — because I wanted to listen to it.