Reason I say 80 years is because before then, pot wasn't really that widely used or known about outside of religious, spiritual and royal circles except for a few small segments of the populace.
chinese emporor shen nung wrote of the medicanal qualitys of marijuana in his phamocopia that dates to 2737 bc. so the chinese have been using it medicanaly for 4000 yrs. the use of cannibis has been wide spred throughout the world for thousands of years. mostly in south africa the middle east and asia. not so much in europe and the americas. keep in mind also that in 2737 bc on up through the renisance era that the only people who could afford to write and print books were the religious sects ie: monastarys, and the rich. it was the avarage person farming the hemp. as a side note most peoples refer to hemp and pot as one in the same until the early 20th century when the us goverment created the name marijuana.

point is people been getting high for a long time, alot of people.

just a little trivia the word hash and assassin are both derived from the same word.