I live in Georgia, so winter is
when I condition.
My season starts in early March. I give my mare November and December of light work and cut her grain. She stays out 24/7, grows some hair, eats a round bale, and gets lightly ridden (and gets brushed up in the arena). I start back around January with actual conditioning. I spend January and February working pretty hard, and am usually good by March.
She doesn't tend to lose a lot of condition over November and December. Two/three days of trails and arena a week keeps her from back sliding too far.