The last few times I've been to MGM, I've always seen him in the Courtyard near the Sorcerer's Hat in the afternoon. He's also in the Stars and Motor Cars Parade at MGM.
The characters show up by the hat several times a day.
If you have a video cam, take it. Herc was a real ham for the camera, posing and making goofy faces. He also asked my kids some pretty funny questions. We were all cracking up (my family, my BILS, my parents.) Probably one of the best character encounters we have had.