I monitor the burn on my Kohler 8KW as best I can by keeping track of hours run between fill ups. It pulls from the port side tank, as does the port engine. The Stb engine pulls from the Stb tank, and I always run both motors. So, in theory, the fuel difference between the port tank and stb tank is what the gen-set burned. I know it's far from perfect, but over time I think it averages out to be pretty accurate. I almost always burn just over .25 gph, right at .3 gph. I allow for .5 gph just to be safe. I've never seen it at 1 gph.
In the summer we generally put about 8 hours per day on the generator, since we run the AC at night. Less in the cooler months.