We have plenty of snow here but we don't *stop* letting our horses out in the pasture. We blanket if needed, throw hay piles out in clean, untouched areas of snow and out they go! Otherwise we'd end up with some basket cases from being stalled for days or weeks on end.
When the grass starts coming up, we have to limit turnout. We start with an hour a day and work up from there. As of right now, they are turned out from about 3pm until 8 or 9pm at night on weekdays and on weekends pretty much all day.