She should be eating about 2.5% of her bodyweight each day. If she is out on half decent grazing (assuming you still have grass) she will be eating about half a kilo of grass per hour. So say she's out 9am - 5pm, she will get about 4 kilos of grass. In summer, this will be more as they eat around 0.7Kg per hour, but at this time of year the grass is a bit harder to come by.
Her bodyweight will be around 550Kg, so 2.5% of this is 13.75Kg. Assuming she is eating 4 kg of grass a day, this leaves 9.75Kg to come from somewhere else. If you feed her a concentrate ration, some of this will be taken up by that. So, if for example you feed 1Kg of concentrate a day, (this is about a scoop of hard feed), then you want 8.75Kg of hay to last the rest of the time.