Feeding over winter depends on breed/size and what else they have available.
We worked on one small bale of hay feeding 40 pregnant Swaledale ewes (for one day) when they had plenty of moorland to forage on. They'd start needing the hay after Christmas, and start to leave it as the grass came towards the end of March. They'd be getting cake as well from early February.
Commercial white sheep will need up to twice as much hay as the Swales.
If keeping indoors they'll need more, perhaps 50% more.
These numbers are based on good heavy bales, about 40 to the tonne.
When calculating your winter fodder requirements, don't forget your lambing requirements. Any sheep indoors to lamb and with lambs will need hay