I have had this discussion with so many people so i am going to start with i am referring to summoners rift not aram.
The rune does less damage than electrocute before 20 stacks as well electrocute has better ap and ad ratio.
In my personel games a regular game lasts about 27 minutes and in my games the average stacks a player has is between 13 and 16
In games that do run late the item will outscale in my 41 minute game of the 3 people who had dark harvest in that game the most stacks was 30 with the lowest 21. That is the point of this rune as it is weak early but can ramp late.
As well a nerf tomorrow will be unveiled that will make it take longer to scale.