how long would it take a larger plane to fall/glide down from say 30,000 feet?


Based on this article about a real incident, I'd guess about 15 minutes. Following a miscalculation in determining how much fuel to load, they ran out of fuel. Using excellent airmanship (the Captain flew sailplanes for recreation), they managed to land the plane without injuries at a closed airport. It's a great story. The plane suffered some damage but was repaired and returned to service. I recall reading recently that it was retired recently.