Let it go as low as it will. This kit will many times stop at a higher point on its own. Mine stopped at 1.010.
Like any kit, it theoretically can go to .990, but I wouldn't count on it.
When the SG seemingly has stopped dropping, give the port a good stir for three days in a row. Make sure its temperature is in the 70'sF. It it hasn't dropped any at the end of those 3 days, it is finished fermenting.