I've seen many instances of this happening. It does seem to fall somewhere after 30 minutes and generally happens as a total body response. In my personal and unscientific opinion - your body releases chemicals, endorphins, and adrenaline while being tickled. This is because your body is confused and is reacting slightly similar to being in pain. The same effect of having pain happens and the sensations will be dulled.
Also, it has been posed many times that the reason for a ticklish reaction is psychological - that it's a playful attack and the anticipation and excitement of not knowing what will happen causes a lot of the reaction. It would make sense that after a long period of the same thing happening, your psychological response would be lessened due to knowing what will happen.
Like I said, I haven't tested these theories with scientific studies, but from my research they seem to make sense.