It really does depend on lots of things, and you have to use your own judgement. With us, I only have a limited amount of grazing and not many sheep-proof paddocks and pens. I also rotate the grazing between sheep, my 2 steers, and 2 horses. This means I can't always put the sheep in the field where there is the right amount of grass for them. You have to judge the quality, as well as the quantity of grass, and also observe the condition of the animals.
If it helps, this is how I approached it this year:
I started feeding hay to the sheep and horses on 8th October, because the grass in both heir fields was looking a bit thin and they were looking hopefully at me every time I appeared. They were all definitely hungry and I had to top up the hayrack twice a day. The ewes also got a few scoops of pasture mix twice a day, prior to tupping (but not the rams as they were on the fat side). I was going to make the Dexters wait a couple more weeks as their field appeared to have plenty of grass left, but then realised they were losing condition so started them on hay and a few beef nuts on 12th and they have now picked up.
Last week I sorted the sheep out. The main breeding group of 7 ewes with their ram went into a new field which had a decent amount of grass; they are barely touching their hay now, so clearly the grass in there is still giving them some nutrition and I'm only topping up the feeder every 3 or 4 days. The ewe lambs (daughters of that ram) have had to go into a very small paddock with very little grass, so they are completely dependent on hay, as are my other ram and his two ladies, who are in a separate very small pen with no access to grass.
I hope that helps!