My Arab has been blanketed at night for three weeks now. She is fluffier than two quarter horses that have never been blanketed. She fluffs right back up after the blanket has been off for the day.
It really depends on the horse and the climate. But my girl has been wearing her midweight for two weeks when it dips down into the 20s and 30s. She's never sweaty or hot when I take it off.