This is my experience, which I offer for your comments, analysis, and guidance.
I am only doing Basic Exercise I and Basic Exercise II from the Mahasi Sayadaw's instructions, e.g. at http://www.buddhanet.net/m_part1.htm
Basic Exercise I is noting "rising," "falling." Basic Exercise II is noting other mental activities that intrude on the noting, and which must in turn be noted.
What happens is that, after about twenty minutes, there
aren't any other mental activities. All that's happening is the rising and falling. The rising and falling occur within a space of inner stillness and silence.
I strikes me that, at this point, Basic Exercise I could go on forever!