"lumbar discs are made up primarily of two types of cartilage-like proteins that are not readily able to heal themselves like other tissue"
Another source say they DO heal:
Ruptured Lumbar Disk
Now what isn't clear is if they are reffering to a vanising of symtoms or actual healing of the disk itself....I guess from a practical standpoint, if you don't have any symptoms, you can consider yourslef "Healed". But if looking specifcally at the rupture in the disk itself...healing appears take years..if at all.
If you have a lot of time...this artilce states disk's never completely heal.
Back and Neck Injury: lumbar back pain and CAT scan results, disc bulge, MRI