Well, since you said "excellent"... if you want to use Federation of Fly Fishers criteria of excellence , in order to pass their Certified Instructor exam, 75 feet is what you'll have to show you can do.
Do you need to cast that far for angling purposes?? Probably not. So in my opinion, that criteria is bordering on dumb.
I think if you can lay out 60 feet, which is what you would need to do to go bone fishing, then you're doing pretty good.
Accuracy is really the most important aspect of casting. If you can consistently put a fly right were you want it, your an excellent caster. In my book, Joe Humpreys is the best example of that.