When to start descend from High Altitude Jet route?

[Jonai Chui]

Thanks for reply, NacelleStrake.

But my question here is if pilot can descend below FL180 on J routes.
As you say, pilots use 3X rule. Likely they reach FL180 at 54 miles from the destination on Jet route, as they have been flying above FL180.
Now can they descend further on Jet route? or need to maintain FL180 until transit to low altitude route?