You don't mention where you are, because that makes a difference, or how old your ewes are. What are you using as hard feed? Hebs don't need an 18% protein mix - 16% is fine.
In theory we start building up feeding towards lambing from mid Feb, as we start lambing Apr 1st, so 6 weeks. In practise we sometimes feed a little earlier than that when snow is lying or the ground is frozen. Once that happens the nutrition in grass is lost, so even if there's anything left for them to nibble, there's no nutritional value there. Hebs will scrape down to ground level through snow, but it's really just something to do.
We are fairly high in the south of Scotland on a particularly windy hilltop, but now our hedges have grown and thickened we are a little less exposed. Our Hebs also have field shelters - part of being a hardy, self reliant breed is finding and appreciating shelter.
I can't be much help with actual weights fed as we work more empirically, in accord with how well the sheep do with however much we give them, and how much they eat quickly. We keep an eye on their condition score and how content they are. We feed nothing like as much as a commercial breed would need, and as said above, a lower protein content is more appropriate for Hebs. We use 'Champion tup mix' which is a general feed (not nuts) for all sheep, not too high in protein and suitable for all ages, tups as well as ewes (the tups hardly get any, just to entice them in at the gather).
So I would estimate that at about 6 weeks before lambing is due to start we give about a small handful to each ewe, then gradually work up to 4 or 5 ounces, later on they get this twice a day, so half a lb, not half a kilo. We carry on after lambing too to help with milk production, until the spring grass is through.
Further south, on lush grass with a low stocking rate and young ewes, you would get away without extra feed, as long as you watch their condition score carefully. In a harsher area you might need to feed more than that.