20' of length 2" from bottom of the foundation, then whatever length to the top of foundation, plus whatever comfortable length needed to make a connection. Personal experiance says more is better. If the foundation is right under, or close to where the panel will be, long enough to make it to the panel. If longer distance away, (like other side of building) I leave a few feet, to insure at least some portion to be made accessible. If not the whole distance. But sometimes the whole length can be an obstical....
BTW when they started requiring it here, there was some confusion early, but was quickly dispelled after the fist few, and people got comfortable with it.
