You have asked an important questions. The traditional answer has been that person gets back most of what they will get back in the first year after a stroke. There is no question that improvement is much faster in the first 6 months and can slow considerably.
There has been a good deal of research on the concept of "Learned Non-Use." This is the idea that the person still has a residual capacities to improve that can be tapped through intensive rehabiliation. This therapy has been called, "Constraint Induced Movement Therapy." The patient is forced to use the impaired limb for up to 6 hours each day. I don't have the space to provide you with the data, but have supplied some links to information on CIMT.
http://www.strokeassociation.org/STROKEORG/LifeAfterStroke/RegainingIndependence/PhysicalChallenges/Constraint-Induced-Movement-Therapy_UCM_309798_Article.jsp http://134.74.134.21/tro/Grotta04.pdf You can Google CIMT and find many more references.
The key is find a physician and therapist to evaulate you and see if you are a candidate for more therapy. Much of this therapy can be performed at home. The dose of therapy is important so a home program becomes very important. There are new devices like the Bioness Ness 200 that also can give a person's hand more repetitions in preforming a functional task.
Try to find rehabilitation physician and therapist who are familiar with these techniques and working with patients who have reached a plateau and want to know what else they can do.
Good Luck
After your stroke you may be experiencing a new normal, but remember what George Eliot said- It is never too late to be what you might have been. You still can achieve new goals.