I am pretty surprised by the times stated to complete a full back piece but as mentioned earlier if you plan to do butt and upper thighs also then this easily doubles the time.
I am having a traditional Japanese piece done from neck to knees which will take around 100 hours to complete.
I am unsure how some people are having full back pieces done in 25 that's pretty speedy. As for the pain its like any old tattoo there are places that make you wince and others that are no problem. I do 4-5 hour sittings at a time as I get twitchy at around 5 hours not sure why possible running out of endorphins.
Perhaps my artist just works slow but I would say if you are planning on full coverage from neck to knees then you would be more in the 75-100 hour range