Is this the longest possible castling? (Chess960)

There are two possibilities with the most squares between king and rook.
Your example with Kb8 and Rh8 : short castling
Second possibility : Kg8 and Ra8 : long castling

Standard rule for the position of the king and the rooks in 960chess is the following. The king must be always placed between the 2 rooks !

The "short castling" is longer when the king is on b8 with the rook on h8 since the king ends on g8 while, with "long castling," the king ends up on c8. King moving 6 square vs. 5.

This indeed is the longest castling as the king has to be between the rooks in Fischer-random.

It looks hilarious though. xD