OMG, knitting is the only thing that gets me through those sessions! If I can concentrate on counting my stitches and following a pattern, I can block out what is going on with the tattoo. That's not to say that my gauge doesn't get a bit tighter, since I'm holding onto those needles and yarn for all they're worth.
I'm only 6 hours in, and I've got about another 20 to go (2 hours at a time). I figure by the time I'm done, my artist will know a lot more about knitting than he ever wanted to.