Well, I'm in process of finishing my full back tattoo piece, but I kind of guess I'm ready to give my experience after two sessions (4 hours each one).
The process was kind of super fast because I had clear ideas and decided very quickly who I wanted as my tattoo artist for this piece, so I went for a consultation day 14 of March, had the first session day 25 and I had my second session yesterday, day 8th of April! My next session is day 28th and then my tattoo is going to be finished :D I'm super excited. So, how much it takes the process? It depends on you and your tattoo artist's availability. In just 1 month I had started and will have finished mine!
About the pain and how it drains you... Wow, man, I can't even describe it with words. I'm a small girl (I'm just 21), but I have a high tolerance to pain. I had two tattoos done before this one: one in my shoulder and the other one in my leg, and they didn't hurt. I mean, obciously they hurt, they are tattoos, but I can't call it "real pain" because I endured it super easily, even forgot about it during the whole session because I was entertained.
So, believe me when I say that back tattoos hurts like hell.
The first session, the outlines, was the most painful one, as it has a lot of detail. 4 hours of outlines killed me. The first 3 hours hurt, but I could even stand it. But the last hour.... The most painful experience I will ever have.
I even cried the last 5 minutes. I didn't move during the whole session and I was trying so hard to not bother the tattoo artist (and I made it, I didn't move at any moment) but it was super difficult and I suffer a lot. Psychologically it was like I was about to die all the time during the last hour.
30 minutes later after finishing the session, back home, I had 3 collapses (I don't know if this is the correct english word: that's when you loose your muscular tone, you fall to the ground... I lost my hearing for about 5 minutes, blurried sight, a lot of sweating, you can't even breath correctly. It's horrible. You feel like you're going to pass out, but you don't, and you stay like this at least 5 minutes). I guess that's because I'm so thin and small.
The second session was just okay (it was just colouring). The first 3 hours were so easy to handle, obviously way more painful than the other two tattoos I have, but compared with the first session it was like a piece of cake.
The last hour was horrible, because I was so tired of handling pain and all the fresh tattooed area (as this is a full back tattoo, is a huge super big area) was super irritated and hurt. So it hurted the already tattooed area plus the area that was actually tattooing at the moment. So crazy pain, but at least I could handle it and I didn't cry, hahahaha.
Anyway I had one collapse again. At least it was just one, not three... Hahaha. The worst about it was that it happened while I was still in the bus and that was super embarrasing.
The most painful areas were my lower back and the areas closer to the ribs. Also the spine, but not as much as the other ones I just said.
I'm even thinking about buying a numbing gel (hush gel) for the next session because I'm so tired of pain right now (it will be like 50 dollars because I'm from Spain and the shipping price is super expensive, but I think it's worth it if I can got rid of the pain...). It would be wise to make the tattoo done in 3 or even 4 months, but I wanted it done in one month because I'm super impatient.
Hope this information was useful!