SchizTech :
Even a long format shouldn't take more than a few hours, so this is definitely a very unusual situation. If this is a new drive, WD should exchange it for a good drive under warranty.
It can take up to about a day. 12 TB / (150 MB/s * 0.833) = 26.6 hours.
0.833 is the ratio of average to max disk speed, assuming a 2:1 ratio of track circumference between outermost and innermost tracks, integrated over the amount of data contained on each track. (The integral works out to 2/3 of the way between 1 and 2, and 1.67 / 2 = 0.833.)
Any format which takes longer means the disk is encountering problems reading/writing. If it's only slightly longer, that just means the disk took some time to map out a bad sector and substituted a reserve sector. But a format taking a week means the drive is dying. RMA it if under warranty, or buy a new drive if it's out of warranty. (And avoid the 5400 RPM WD drives - they have a head parking issue which can cause lag and stutters. That's the Green, SSHD, and most of the Blue WD drives.)