During all times of the year they can vary by resort.
Whatever, hands down the best pool hours are at the Boardwalk - it's the onlyDVC resort pool that is officially open 24/7 year round. The life guard stations are generally staffed from 8 to 10 (the same hours for the slide; it's only open when the guards are there), but guests are allowed to use the pool at all other hours. Trust me, it's wonderful to be able to take a swim (or a dip in the hot tub) after returning from a 1am MK closing.