Can posterior vitreous detachment last for a year or more?

Yes, it can: A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. A common symptom is flashes of light (photopsia). There is a small risk of a retinal tear / detachment. Duration of PVD is usually 4-6 weeks, but can be sudden (trauma), days (after cataract surgery), months, or rarely a year or more. Nearsighted people tend to get PVD earlier.

It last for life.: Posterior vitreous detachment (PVD) is a natural phenomenon that occurs in all people sooner or later. The longer the eye is (the axial length) the sooner it may detach. If a thorough eye exam rules out any tears in the retina it is nothing to worry about.

Forever: Posterior vitreous detachment is a natural process of the conversion of the gel vitreous to a dense syrup. This takes many years and produces a permanent change which is usually benign. So it last a lifetime.