How do we know posterior vitreous detachment has completed, is there any way to know this, i mean in young high myopic like me, will it last forever?

Yes: Your ophthalmologist will be able to know if the PVD is complete, but you may well have symptoms forever.

Not forever but slow: The solid gel vitreous changes to a dense syrup once it starts over about a 25 year period In most cases while there are sometimes symptoms like floaters, and light streaks, the outcome is benign. Rarely there are strong attachments to the retina which can be threatened. If you have continuous symptoms or loss of vision, see an ophthalmologist right away.