I dont remember how long it took until it stop. But it took some time. I would say i felt this annoying sensation at least some 10 (or maybe 20) times after deciding it was only into my mind... and i had to keep saying each time that it was into my mind. Eventually, it stopped. Anyway, the most important is not to worry about this. Your mind is greater than a mere sensation, so dont let it overcome you. Show this sensation who is the boss, and sooner or later it will leave.