If the spine itself gets nicked? Never. As a general rule, injuries to the spinal cord are something you never really recover from. Minor abuse can heal, but severe injuries? Not so much. There are exceptions, but, as I’ve said before, I’m not a medical expert, and I don’t know exactly what governs those.
If it’s just tissue trauma, you’re looking at a couple of months laying around, with her back sewed up. But, if it’s severed nerves, or damaged the spine, those are going to be things she never gets back.
I don’t usually just add responses into the original text without comment, but here’s a couple of details I didn’t cover, with thanks to both jhinnua and kingpainasylum.
@jhinnua said: Also if it heals to a point where she can recover there’s a high chance of reinjury from muscles not healing in the right spots. And periods will be extremely painful for a long period afterward (providing she has feeling down there at all)
@kingpainasylum said: Severed nerves would affect her sense of touch, possibly her back would be numb and feeling tight even after the wounds have healed. And if it damaged the spine, well, speaking vertebrae, it would affect her ability to walk; paralysis. Good luck.