How far away from you does grass spread?
Grass spread is controlled by "random ticks". These happen when the center of the chunk section (16×16×16) is within 128 blocks of a player. That means that grass spreads in a pretty blocky sphere around you, with the closest point being 114 blocks away from you and the farthest 142 blocks away from you.
But I wouldn't recommend building a sheep farm this big anyway, because the tens of thousands of sheep would definitely lag the server and your client.