OK, I'll start off by saying I've seen a good few Roman sites over the past several years. So I have to say that Orange was not one that I would recommend. The Ampitheatre is impressive in its way, but I suggest going during the Choregies, which is a week or two of large choral performances that offered here during the summer. That way you could enjoy the music as well as a quick tour before the show. And the Arch de Triomph here is likewise not worth it. Save your time for better Roman monuments in France, at, say, Arles. Still, if you are passing through anyway, you could stop a take a quick look. But don't give Orange more than 2 hours (unless you're here for the music).