Some resorts do not open until July. Others close in September. Generally the farther south the area, the later the area opens and closes. Some years snow does not fall until the end of June. Planning a vacation in August is generally safe.
Most Chilean areas are above the treeline high in the Andes.
Unlike most European and American resorts, Chilean areas do not descend into valleys. Rather one must drive up windy mountain roads to get to the areas. Snow storms generally last 2-4 days and are accompanied often by wind. Ski resorts often limit operations or close during snow storms. It is a good idea to spend more than a week in Chile on a ski vacation to hedge against possible bad luck with the weather.