But how low can the temp fall after a temp setback of 8 hour?

A:  Short answer, it depends of lots of factors (outdoor temp, wind speed and direction, presence of sunshine or not, quality of house insulation , etc.). 

In fact, generally speaking, with an AI based system like the SmarTHERM, the temp will rarely fall for more than 5°F (3°C).  By rarely extreme cold weather (-15°F (-25°C)), the fall may sometimes reach 12°F (7°C).  (Taking into account that the SmarTHERM system will restart the heating far before the 8 hour setback time is reached).