I got a 14 hour 1/2 sleeve on my upper arm in 3 sessions, a month between each. The timing was my artist's choice, not mine and I'm glad. 4 hours at at time was enough for me. Not entirely the pain involved, sitting/laying for long periods, being tensed, etc. I just find it to be more enjoyable to do shorter sittings, I have nothing to prove. Also, some tattooers have a heavier hand than others.
Couple things that I'd add to Dan's list. No ibuprofen, aspirin or other blood thinners before (of course unless it's something very necessary and then I'd run it by you doc and/or let your artist know.) If you're traveling very far right after, I'd suggest stopping for a quick meal. Even with a snack during the tattoo I felt much better refueling before the drive home.
Best of luck and let us see how it turns out.