Now that I'm back to freelance, probably 10-14 hours a day
But definitely listen to your eyes and rest them when you need to. You can also download one of those screen tinters that turns the screen warmer so it's easier on your eyes. (At the very least, when you're not drawing). Turning down brightness would help, too.
Another thing I would suggest is getting an eye exam. Having proper contacts or glasses can make all the difference so your eyes won't have to work as hard. I wear glasses, and recently my eyes were suddenly getting worse and just strained much more easily. I went in expecting to need a stronger prescription, but it turned out I needed a lesser prescription because my glasses were too hard on my eyes.