Transit by Earth lasts for 13 hours = 0.54 days

  It occurs once per year

  The fraction of time Earth blocks the Sun from the point of view of
      any observer (in the ecliptic plane) is about 0.00148

  One would have to observe about 1/0.00148 = 675 copies of the Solar System
      in order to have a 50-50 chances of catching one in the act of
      transiting

  One ought to record at least 3 transits to claim a planet, though
      I think 4 or 5 would be better