I have not hiked it personally, but this is what my map says:3 hours from Asahidake to Tenninkyo or 2.5 hours the other way around
That is a considerable distance for an unexperienced hiker.
it is a calm course without any other hikers around
In other words: no help around if you get into troubles
And most importantly:
a section of the trail (near Tenninkyo) was closed in December 2007 and had not been reopened by 2008 when the map was published
Inquire in advance whether the trail is fully passable, otherwise, you might be forced to backtrack 3 hours after a 3 hour hike.
There is also a risk of meeting bears, so you better bring a bear bell.
To unexperienced hiker, I recommend not to use the trail, but instead walk along the shorter trails at the upper station of the ropeway. There is a risk of snow around the upper station of the ropeway from around mid September (it is the first place in all of Japan to receive the first snow).
