This lady wrote a great little piece on blanketing. Go here to read the whole blanketing 101: http://barnmanagerblog.blogspot.com/2011/10/stable-sheets-and-heavy-weights-and.html
The shortened version of how you should blanket is as follows, but please do visit her blog and read the whole explanation of all the terminology and why you should blanket when and with what, plus when you should layer.
55 and up: no blankets
54 to 45 and/or rain: turnout sheet
44 to 35: turnout sheet during the day, stable blanket at night
34 to 15: medium weight
15 and under: medium weight over stable blanket
Now with my kids, they get a turnout sheet 24/7 when it drops below 55. We have sufficient shelter out there, they get plenty of forage to cook in their bellies to keep warm, and when it drops in temperature, I start layering either underneath or over the top. I have fleece liners, turnout sheets, and medium/heavy weight turnout blankets (because I don't keep my kids in a stall at night anymore unless the temps are ridiculous and/or rainy).