The mountain is "closed" outside July and August, and they put scary signs up with skull and cross bones. Just ignore them. Of course, there are no huts and refreshments this time of year. On a nice September or October weekend there will be a few people on the mountain.

Regarding starting from the bottom, this German guy sprinted up the mountain in 3 hours and 29 minutes. But then he falls at the top and his skis go sliding down the mountain.
It took me like 7 hours from the 5th station Yoshida. It was tough with the soft snow. Crampons did not help. Then less than 2 minutes to ski down.