There are no specific time requirements to unlock "expert" staff ghosts in Mario Kart Wii. The ghost system works differently than many players assume.
Ghosts are labeled Beginner / Intermediate / Expert based on the original player's performance when that ghost was recorded. You don't "unlock" Expert ghosts by hitting a time target—you encounter them by racing online against skilled players.
Race more, race better players, and your ghost library will naturally fill with Expert data.