For statistical purposes only, I charged my ESV to 100%, used it as normal (6-8 hits at lunch, at 335F & 355F), then checked the charge. It was down to about 85%. I unplugged it, and used it the next day, and checked it that evening, and it was at just about 75%. Used it again today, same usage, plugged it in, and it was down to about 60%. I'm letting it juice up a bit, right now.
So, really, I'm using about 15% of the power every time I use it, and, to be more precise, I hit it at 335F for maybe 3 hits, bump it to 355F, hit it 3 or 4 more times. I might "stir" if I think it needs it, if by "stir" you mean open it up, poke a fingertip in to see if it's all brown on the bottom, ruff it up a bit, then stick the lid back on. I often don't, as I'm at work. Much less smell possibility if I leave it closed.
It doesn't seem to be using more power as the charge is reducing, either. YMMV, of course.
So, based on this, I could probably wait until Friday to charge it, but I'm satisfied with just knowing that I could possibly eke out a whole work week on one charge, if necessary, especially if hit lightly (3 or 4 per day).