Welcome!
The length of your session depends on several factors. Others around here can add more.
- The size of the tattoo. If it's small, couple hours, it's easy to sit through in one session. If you don't, you will be mocked and ridiculed. Maybe even to your face. If it's larger and your tattooer has the expectation you'll sit for the whole thing in one sitting, you do it or try your damnedest.
- Your tattoo artist's scheduling methods. Some only schedule in well defined time slices, for example, 3 hours. If the tattoo takes longer than that, you sit multiple sessions. If your tattoo artist schedules open-ended sessions, then you'll go until he finishes whatever goal he set for the session. This could be time or a particular part of the tattoo. Or, he'll keep going until you absolutely can't take another second and you have to tap out. No one likes to tap out. It's a sign of moral weakness, contributes to global warming, and will cast poorly upon your family and children. Even after many hours, it's so psychologically awesome to hear, "Ok, we've reached a good stopping point" instead of crying silently into your hand and whimpering that you're sorry you just can't take it any more. The silence that follows such an abject admission is enough to curdle milk and paralyze small dogs.
- The body part. This thread can shed some light: http://www.lastsparrowtattoo.com/forum/general-tattoo-discussion/206-most-painful-spot-get-tattooed.html. Personally, I can't speak to many body parts because I'm only working on a back piece, but yes, location can play a role. (Although your back, in terms of pain, is it's own set of differing body parts.) My one and only tap out was after 5 hours on the top/middle of my back that included spine down to the bottom of my ribs and the bones of one shoulder blade. I've had many sessions longer than this. Apparently, many think the shoulder blade isn't bad. My next session is to do the second coat in that whole area. I'm queasy just thinking about it.
- Your own preparation. How you prepare for your session has a big effect - some may argue that good preparation plays as big a role as where on the body you plan to get tattooed. If you go in tired, hung over, or on an empty stomach, body part may matter very little.
Anything else to add, anyone?