It depends. If you are going to run physical then I would say at level 59 after he learns Wild Charge and Coil. If you are going to be using special then I think you can evolve it whenever you want, he can learn his moves using TMs.
I think you don't really need to worry about it. Eelektross can learn his best moves as an Eelektross, via TMs and level up(use the move remeberer).