Midweek Mammoth just runs the core lifts around the Main Lodge. But close to 50% of terrain is accessible if you know where to traverse. Last weekend they also ran chairs 4, 5, 10, 12, 13, which make about 70% of terrain accessible.
When and where to ski is normally a function of weather and when ungroomed terrain softens up. Grooming is excellent and they will salt the groomed runs if it's really warm. If there are any weekend crowds they will be at the mountain access points chairs 1&2.
Weather forecasts show some storm activity this week with high snow levels ~9,000 ft. This probably means rain in town and a few inches of very heavy snow on the mountain. The off-trail skiing may be unconsolidated cement until we weekenders (I will be there next weekend May 7-8) pack it down.
Normal clear spring weather with overnight freezing brings consistent spring skiing. And with the massive base and grooming/salting it really will be worthwhile into July most of the time. Worst case scenario is a cold/windy day when nothing softens, but even then there will be quite a few manageable groomed runs.