To me the length of the hair depends on the lake im fishing to how wish the fish are and what everybody else is using. Sometimes i used rigs with short hairs by that i mean 5mm away from where it comes away from the hook and up to hairs of 3-4 cm in length.
When it comes to d rigs i like it quiet close to the ring but with a slight gab so when it is sucked in and blown back it acts like a hinge, the bait goes backwards and the hooks is forced downwards
I also also think that it depends on the size of the fish your targeting...