id imagine it depends on the shield...some are stronger than others, unlike rebellion, there seems to be many varieties of shielding available...ranging from the small-scale base protection of hoth, to global shielding like that around Ukio, to the complex, layered defenses of coruscant. basically, there is no set formula to taking down a shield system, but in general, the richer and more developed the planet, the tougher it will be to knock it out. the generic shields of rebellion however, take something away from this to the point where it basically comes down to, the more generators you have, the stronger it will be.