I don't get big lulls of no drawing - even at the most drained, it'll only be an hour or two before I start doing thumbnails or thinking about visual art. About 4-5 hours is the most I can crank without a break on artwork, but these days that can be very varied, and penciling or inking doesn't have any drop, but painting specifically does. I think this might simply be scope of the project and experience (like I haven't really tried to do big comic book pages much). As I learn more about painting the time I can crank on a single painting is pretty consistently 2-5 hours (I pencil or ink things separate from Photoshop, traditionally because I find it easier to work with), and after that, that's when I can feel really crushed, really easily. I'm not sure how much is a human thing (everyone feels drained walking away from longer chunks of activity) and how much is a personal thing (lots of self-judgment fairly independent of actually making pictures).