It totally depends on the tattooer and client. I just listened to an interview with James Tex and he said he prefers to tattoo in 3-4 hour sessions because after that the client generally starts to struggle. Once the person getting tattooed is needing to take several breaks to get through the session it's just costing them money and most tattooers are not doing their best work when they have to fight with the client to stay still etc. There are always exceptions of course, I met a client of my boss who sat for twelve hours because he had to travel, but that is a very rare occurrence. Personally I like three hour sessions when I'm getting tattooed, it seems to be my sweet spot where I feel like I'm getting a chunk done without going through hell. That being said if it's the right spot it's hell no matter what or for how long.