How long a leash?

Ok many CG's allow pets, great. Most if not all CG's state that pets in my case dog must be attached at all times, that I can understand as some people have no control over their dog.
So my question is when not walking with your dog but at the TT site how long of a leash/cord do you attach your dog with? Do CG's limit the length, is there a standard?

Thanks