That would be about 1 ounces of pure THC (about 28 grams) - the THC equivalent of about 750 joints. LD50 of alcohol is 13 shots (in 15 minutes), or 19.5 oz of 80 proof liquor (about 8 ounces of pure alcohol). The ratio between effective dose and lethal dose is dramatically higher for marijuana, making accidental death much less likely.
The Merck Index lists the LD50 (dosage of a certain chemical which will kill half of a population given that dosage) of pure THC to be 42 mg per kg of body weight for rats when the THC is inhaled. LD50 for monkeys is 129 mg/kg (about three times higher). Assume we have a 75kg person (165 lbs) and that this monkey LD50 also applies to humans, that gives us 9,675mg or about 10 grams, which is more like the equivalent of around 270 joints (in about 15 minutes).
With marijuana itself, it does not seem feasible to take a lethal dose, but with 80-90% THC extracts (apparently, distillation methods can now produce better than 99% pure THC) it would be more feasible. I don't know if passing out would self-limit administration that way. I would guess that most people would pass out (or just forget what they were doing) before taking so much.
The oral LD50 of pure THC is 666 mg/kg in rats. So if it is the same for people, that would mean a 165 lb man eating about 50 grams of pure THC, or maybe thre times as much if the same rough ratio exists between rats and monkeys (and presumably humans). That would mean eating about 5 ounces of pure THC, or about 6 ounces of the commonly available highly concentrated oils.
Bottom Line: With marijuana itself it seems unlikely to get a fatal dose of THC, but with concentrates, the LD50 is probably a few hundred (up to 1,000) times the effective dose (within about 15 minutes).
From the CDC:
