Yes, it can take a while for your kidneys to repair. One of the Experts has said there is data showing that renal changes are still present until 6 months postpartum.
Technical explanation: Once your body mops up the soluble Flt and your unbound VEGF levels return to normal, the VEGF can go back to shepherding the repair molecules into the holes in the blood vessels that developed during pregnancy, and eventually the damage will be repaired, but depending on the amount of damage, this takes time.
It is especially likely for people with some sort of underlying kidney disorder or whose kidneys took such a severe hammering that some of the tissue necrotized to spill protein postpartum. Most of our posters regain full function, but I know we have had at least a few women who didn't get off dialysis. On the other hand, I never needed it, and Catherine only needed a few sessions before her kidneys kicked back on.
One good reason to have a preconception appointment with a MFM is to run a 24-hour proteinuria and get a baseline value. Plus there are some blood pressure meds that can only be used outside of pregnancy but that can lower your proteinuria, so women with elevated baselines can take meds to lower their proteinuria before conceiving.