Depends on how long the hair was to begin with and the severity of the
injury to the skin. A few weeks, probably, to be back to a full length.
Longer for a long-haired cat.
If he badly injured the skin, parts of the coat may or may not grow back
the same. Fur doesn't always come back over scar tissue, at least not
fully. I've seen a few animals grow back white hair over scars as well,
regardless of original coat color.

It will take several months.  You are lucky that the cat is not bothering
its tail.  One of our cats bit his injured tail repeatedly so we had to
put him in an elizabethan collar in order for the tail to heal.

When my cat, Nicholas, was shaved in several places for a medical
workup (no injuries, though) it took about two months for the fur to
grow back normally. (A little more in some areas, a little less in
others.) He's a shorthair kitty. The hair grew back the same on his
ruff, but it grew back wavy on his leg, and slightly longer on his
back (the area that took the longest to grow back) such that it now
parts a little on either side of the base of his tail, which it didn't
do before.

From what I've seen among the cats at the kennel I work at, regrowth
time seems to vary quite a bit from one cat to another.