an average 4-5 foot plant of decent maturity could yield 3-4 ounces, thought there are a lot of factors like the strain,and bushyness plays a big role, if by fall your plant has lost half it's leaves from under nitrification (is that a word?)then yeah you could get less than an ounce from an average plant, i had a 5-6 footer one year get discovered by kids, who picked every single leaf off it, after i transplanted it and let it grow i ended up with one tiny top cola that weighed about 7 grams dry...
lets see, an early frost will definatly kill your yields, 3 years back i got only half an lb from 9 very bushy 5-6 footers, because of early frost, i was predicting a quap per plant till a freek two day frost in early september killed them.
it's best not to figure that out till after your done with trimming i have found, soooo many variables, and the more you expect the more you get let down when say a ripper takes all your plants 2 days before you planned to harvest. hell if i woulda calculated my yeilds 2-3 months ago ida said 20 pounds, im glad i wasn't counting on it because now i think the most ill be able to pull is one pound, maybe two if i have an amazing rest of the season.