Hay, straw, sileage etc is the absolutely essential part of a ruminants diet. So the 2kg bit is a a guide, not a master. Therefore, if you have 12 sheep then one roughly 25kg bale of hay will last them roughly a day. But if they've finished it by early evening, then you give them some more - don't leave them all night with nothing. Similarly, if a good part of the bale is left you don't shove it down their throats till it's gone because they haven't eaten 2kg.
Big sheep obviously eat more than smaller ones. Good qualility soft hay, full of herbs, that smells so good you could almost eat it yourself, will be wolfed down, while stalky stuff with no smell will be picked at.
The animals will eat more on a cold day to keep warm, and less in a mild spell, especially if they have access to grass which may have grown a bit.
So - look at your animals - they'll tell you if 2 kg is enough, and if they eaten up in less than a day then it obviously isn't!