Does the myelin sheath regenerate itself? if so, how long does it take?

Myelin can heal: Demyelinating diseases like multiple sclerosis create a situation where the myelin will not heal itself. Traumatized nerves do heal but they only move a few mm a day so it will take time for the nerve ends to meet and repair themselves. The myelin will heal in time at east several months.

No definite: Unfortunately while axons are known to grow at an approximate rate of 1 mm/day , there is less known about myelin rates of growth. If you like research papers try searching online for "axon-glia interaction and membrane traffic in myelin formation" by robin white in frontiers in cellular neuroscience. It's technical but provides summary of what scientists are exploring now.