If your training 12 sets hard core per muscle group how many days will it take for that muscle to fully recover so I can train it again?.
Define "hardcore."
I would consider "hard core" at least training to failure if not beyond (e.g. with forced reps, cheats, etc.)
12 hard core sets, then, for instance, would be three sets of pulldowns to
failure at 12, 10, 8 reps, then three sets of BO rows with maximum weight, each set to failure (say 8-10 rep range), followed by three sets of weighted chins to failure at 8 - 12 reps, followed by three MORE sets taken to absolute eyes rolling in the back of your head, bleeding out of your nose, feeling like you're gonna black out, death-screaming from the barrel of your chest and the pit of your gut, feel like you're the gawd-damn juggernaut (BITCH!) failure of bar-bending rack deads.
I think 12 sets is a bit much, TBH, if you're going by what I might think of as hard core. (Hard core is def. relative, too, of course...)
This is not to say that you couldn't do 12 sets and have a hard core workout. (E.g., doing something that was German Volume Training style by 12 sets could make you want to cry for your momma.)
If you're really just trying build your back, why not experiment with *your* 12 sets:
Start off with just 2 or 3 days recovery for two - see what happens. Take a break for a week or so, then bump to 4 days in between workouts for a couple months. Another break and then 5 days recovery for a 2 month period and see how you feel, grow and gain strength...
-S