Don't think many of us have had a radial nerve severed,
but many have had PN after surgery.
Sometimes it just doesn't happen (nerve regrowth).
Most observations is that regrowth, when it does happen,
is extremely slow.
Painfully slow, (pun intended) as the nerves tend to send mixed signals of a pain type to the brain, during the process.
Dependent on the type of nerve, and proper supplements and diet, it could take up to 2 or 3 years for a gap to be bridged.