I assume you refer to Mount Aso's Nakadake Crater (as there are at least 29 peaks named Nakadake in Japan).The current timetable is as follows:
Departures from Aso Station:
9:05, 9:52, 10:40, 12:20, 13:25, 14:15, 15:20
Departures from the ropeway station:
9:50, 10:45, 12:10, 13:20, 14:10, 15:45, 17:00
The one way ride takes 40 minutes and costs 540 yen.