I don't know what the research says but from personal experience (my teeth) it seems to be working for months. I had really bad sensitivity on my premolars from recession and since I used Gluma once I haven't noticed any sensitivity until recently (almost a year later).
I have no idea how long it's supposed to last. I would imagine lots of things affect the length of time (individual variation, way it's put on, where in the mouth, etc).