If there isn't a heavy frost in September or there is mostly clear sunny days the leaves will last a bit longer and can vary by up to week or so. Generally speaking the leaves of large trees usually turn around the same time each year, as the leaf color is related to the amount of sunlight. If it stayed frost free and warm for the next two months the leaves would still turn around the end of September due to our latitude.
That said, the leaves of some shrubs and plants rely less on sunlight and could last longer. Lilac bushes can go a long time if there are no freezing temps.