The AM is usually a loopstick inside the radio. That is for FM. The length really doesn't matter much.

On AM, the loopstick antenna is part of the tuned circuit and is sensitive. On the other hand, FM pretty much tolerates anything. Unless you want to find the correct ground and impedance and make a proper antenna like those T type ones, just don't worry about it. One wire going to the screw that is the farthest away from ground, any length you like.