Hucho's book provide an example for this.
The back of the insight has roughly the same shape as the Mercedes C 111 III, though shorter and thicker.
On the attached image, the original shape they started with was approximately one third the lenght from the rear wheel skirt.
They tried different lenghts from 0 to 1.5 meters, beyond which there was no improvement in drag.
The function between boat tail lenght and drag reduction in this case was not linear and went up to a 25% reduction in drag.
They settled for roughly 60 cm which lead to roughly 18% in drag reduction.