I can add to the tank sizing. I use a converted 30 gal gas tank (w/10 gal of antifreeze/corrosion control) and on a cool day, after 90 minutes of 2500w load, the tank is about 175F. About that time, the batteries are full enough, and I shut down for 12 hours. As it gets hotter, the tank will radiate more heat, and more will steam off the top, but I would guess 2 hours top, at 2500w with my 6/1 at 1,400' elevation. I've not run much longer, since I'm trying to keep runs at high load to prevent carbon fouling.
I do find a lot of dry fluffy carbon building up under my exhaust outlet (I have a angled plate to divert ex away from the shed) and the plate gets a 1/4" build up of fluff on it.