the AHA survey in the 2018 January/February zymurgy magazine has the total homebrew per at 43.5million gallons. It appears to based on 1.1milllion American homebrewer and an average 8.3 5gal batches a year.According the forum home page footer there are 179,198 members on HBT.
Using the 2018 total that's a 0.3360964 liter per year per member average.
Someone clearly isn't pulling their weight, dammit!!