You would have to smoke 682 kilograms of marijuana in the course of 15 minutes to overdose. However the logistics of purchasing that much of the drug alone is insane, let alone trying to smoke it in 15 minutes so, chemically yes it is possible to overdose via marijuana toxicity.
Marijuana is not a drug that can affect you so bad that it can cause you to overdose. Marijuana is a substance that messes with you mentally, not physically. No matter how much you smoke it does not affect your body in any type of way, which makes it entirely impossible to overdose.
Marijuana is an unusual drug in many respects. There are certainly unpleasant side effects possible from marijuana use. And people can definitely become addicted to it, to the point where it negatively impacts their lives. To date however, there have been practically no reports of overdosing in the sense usually associated with alcohol or other illegal drugs, where the amount of the substance itself turns out to be toxic enough to kill you.
Its not a drug. Its a natural grown plant on this world. Now when you go mixing it with other "drugs" then yes. But by itself Marijuana is safe. There is nothing about it that can kill or even harm you that im aware of. So smoke away and have fun.
It is impossible to overdose on marijuana. I know because i have personally tried and know many others who have. Do you think that If you could overdose on marijuana there would be people fighting to legalize it. In fact there are states that legalized it already and I haven't heard any stories about people overdosing on it yet.