Law 6 (The bat)
1. Width and length
The bat overall shall not be more than 38 inches/96.5cm in length. The blade of the bat shall be made solely of wood and shall not exceed 4 1/4 in/10.8cm at the widest part.
There is no regulation limiting the length of the handle, except that convention holds a short handle at 11 inches. As a regular bat is generally no more than 33.5 inches, so you can safely say a long handle may be between 11 and 14 inches in length, but for mass production's sake (and considering the fragility of a handle) you'd find a typical long handle to be around 12.25 inches.