I'd say you have about a week/maybe two, until they close down highway 20 at the Ross Lake Dam trailhead (seasonal closure for avy danger). You can still roam the park, you just can't access any trailheads pass that point by motor vehicle.
Ross Lake is a beautiful, lonely place this time of year, lots of lower elevation trails/camps. You might consider that if the first big snow dump has already happened up high.