I like between 10 and 12", I use the longer fades to shorten up the working limb, if the bow starts to take set I can always go back into the fade for some more working limb.
Every time I do this, I have trouble matching the thickness as I remove the length from the fades and end up with a slight hinge just off the handle.
Otherwise, I just adjust it to the bow. If I have plenty of length, a Perry reflex or anything else that protects the limb from strain, I might make nice long fades that start where the grip ends, maybe a 2-3" high cutout shelf, and have a total length of 12". If I'm cramped up, worried about the stave, or whatever, I might have the handle grip actually be part of the fades, and start the upper fade right above the arrow pass, ending up with as little as 6-7".