If you wear glasses long enough, will your vision improve?

Unlikely. If used for near sightedness or farsightedness or astigmatism, then if vision does improve, chances are it is unrelated to the glasses. Glasses help compensate for poor vision, but do not treat the underlying problems that lead to poor vision.
Unfortunately not. The only situation in which glasses can actually improve vision is in young children being treated for amblyopia (lazy eye). If given glasses early enough and the other eye is occluded, the vision will actually improve. In older children and adults, glasses simply improve the focus that the eye cannot do on its own. Wearing glasses will not weaken the eyes either.