The quietest time is probably January, it's the cheapest too. There we virtually no queues at all. There are however a few drawbacks. One is the weather it can be cold, we went last Jan and it was cold, but not as bad as i thought whereas the year before it was laden with snow. Second is the fact they do maintenance on some rides. Take a look at this list which was for Jan this year.
My midweek mid-late November Christmas season trips to DLP have been the quietest of any trips we've taken there, including February and March. Plus you get the Christmas themeing (which will take your breath away its so beautiful ). Milder weather generally than January and February too.