A hugger should extend just over the verticle position of the tyre, but all that I have looked at lately are much shorter and as has been said are "bloody useless" other than to reduce road grit and filth spatter onto the rear shock. The other day I saw a bike in a dealer's that had a home made extension fixed onto the branded extender on the front guard.
What with the fad for a tail tidy, the only way to keep your back dry, on most bikes, is to carry a pillion passenger.