It was -1 last night is decided to stick a fleece on my unclipped 2 year old fjord whilst he was stabled overnight. He was slightly sweaty this morning so I certainly won't be doing that again unless it's even colder! During the day he's unrugged except for if it's wet and windy. He is coping well so I'm just having to control myself from overrugging him!
As others have already said - it depends on the horse. Also IMO it hasn't got properly cold yet - the long range forecast seems to be for a particularly hard winter.
I've got a fully clipped warmblood mare who is warm in her 150g turnout whilst my blanket clipped warmblood gelding is in his Duo with 200g liner and neck cover.
Unclipped retired cob lives out - no rug
Clipped (chaser) trotter type lives out - middle weight
bib clipped welsh cob - lightweight when turned out lightweight stable when in
There is a range of rugging going on at our yard, from unrugged to heavy weights. My girl has an irish clip / trace clip mix, stabled overnight and both stable and turnout rugs are 200g. She is a slightly chilly first thing in the morning and probably needs a little bit more, but I prefer slightly chilly to sweating and she would probably start sweating with the 300g.