Good Afternoon, Mainecoonmaniac,
Since the chemical cost is nominal for B & W, I agree with using at least eight ounces. Normally, I use ten or twelve ounces for four sheets of 4 x 5, probably a good deal more than necessary. Using much more than that in my Chromega drum can lead to some spillage if the drum is tilted a bit when I reverse it every thirty seconds. A good starting point for time is whatever is recommended by the manufacturer for continuous agitation processing.