shell regrowth question

[Logz23]

How quickly does a tortoises shell usually grow back? also is there anything extra you can do to accelerate this growth?(safely of course)
the sulcata i found has about a 1.5in x 2in piece missing.

 

[tortoiseluvr]

I am not sure if the shell will grow back completely. My box turtle was attacked by a dog and part of his shell above his hip chipped off, and the vet said he would always have a chip even when the wound heals.