Does the average American smoke pot at least once a week?

I don't know if I've ever seen any estimates of how much marijuana is consumed annually within the US, but this number hints at it:

Since the beginning of 2007, the report states, Mexican security forces have seized about 65 tons of cocaine and more than 9.3 million pounds of marijuana.

Assuming all the weed was destined for the US, that means that about 7.5 grams of weed were inderdicted for every man, woman, and child. A rough estimate – the marijuana was likely consumed in Mexico and Canada as well as the US, but then again both Canada and the US have non-trivial amounts of domestic production. This amount of weed is a bit more than the street equivalent of a "quarter" (aka, a quarter of an ounce), and is roughly enough weed to get one person high about 15-40 times, depending on quality and tolerance. I would be very surprised if even half of all marijuana produced was seized before it reached smokers – I would put the number at about one-quarter, which would mean that the average American gets high at least once or twice a week. But this seems a bit unrealistic...did I mess up the math, or are my suppositions incorrect? Or do Americans just smoke a lot more weed than we realize?