I've never gone looking for studies, but it's an experiment you can conduct on your own. Just start adding hot sauce to your food, then gradually increase the scoville rating of the hot sauce you use. Barring any sort of medical condition (allergies, etc), the progress of your tolerance will be obvious.
Now the part I don't know is whether there's any sort of maximum level of tolerance for humans, or whether a dedicated individual can potentially Dragonball Z their way up to the point where OC loses its effectiveness. That's where we need a study. Good luck finding volunteers, though. Also I have no idea whether building up a dietary tolerance will have any effect on exposure to other parts of the body. Ask any Thai food lover who's ever forgotten and rubbed their eyes after eating, they'll tell you.
--Patrick