Alcohol is one of the easiest drugs to OD on, and one of the commonest. There are no reported cases of OD'ing on cannabis, ever, at all. In elementary school my DARE officer told our class that a person would have to smoke a pound of pot in an hour to overdose on THC. I don't believe he was citing hard science there, but his point is clear. Even the anti-drug people admit you cannot overdose on herb.
You will not overdose on marijuana.