I do not drink coffee, but I will respond to the basic question, "How tired is too tired to paint?":
When I was at the height of my painting madness, I was never relaxed enough to dip a brush in a crazy place. Waaaay too focused and serious.
No matter how tired I was sometimes, I felt driven to work on a painting that I had started. Not out of joy. Not out of inspiration. Not out of freshness. But out of total obsession to work on it towards the goal of making the final realization appear in physical substance.
I used to be a local fitness guru, and after i worked out for hours and taught my classes, I would come home to my little efficiency appartment where I had a matress I slept on leaned against the wall with my homemade easle propped against it. I would skip eating, and go straigt to the task of obsessing on my next details, sometimes until early the next morning, ... totally oblivious to fatigue, hunger or mental exhaustion.
I had a lamp that shined on my painting in that appartment-- it was like a spotlight shining to some greater revelation. Sometimes during the heat of the creative process and in the summer heat of the un-airconditioned atmosphere, I would get so charged over my inability to resolve the painting that I would throw brushes against the wall in frustration.
Obviously, I was not too tired to angst. So, I rarely was too tired to paint. Lord only knows what would have happened if I drank coffee, with the extra caffine and all.