Here is a side by side pic I did years ago of two publicity photos to make it easy for everyone to tell the difference.
He can be seen wearing it the most in the museum after saving the large pot that Robin crashed into. Other than that there are only a few frames that remain throughout the film. On the DVD extras he can be seen wearing it a lot while they're filming in the museum.
The whole suit was made for Val, but when Val didn't return George was cast in the last minute. Principal photography had already started when it was decided to sculpt a new cowl for him, the main reason they did was because the Panther cowl did not fit him comfortably around his throat and it affected his speech.
All the stuntmen wore the Clooney Panther cowl throughout the film, they didn't reshoot those scenes after the new Clooney cowl had been made because it was far more important and less expensive to just reshoot the scenes with George.
A repainted Clooney suit with a Clooney Panther cowl was later re-used for the OnStar Batman Commercials and the pilot episode of Birds of Prey.