Judging solely on this one photo, I would say the mare is a fair ways off yet. The bag needs to fill considerably on both side, and closer to when she's ready, the nipples will be pointing more outwards and will be more "spread out" in appearance. You can also lift the tail and kneel down behind to get pics from the rear. As she gets closer, you'll see the bag growing in size, getting fuller, firmer and hanging lower.