House: built in 1912, central Ohio. 2 story, 2000 sq ft. w/ newer insulated windows but minimal to no insulation in walls.
Furnace: 90+ nat. gas forced air, 3 years old, correctly sized.
Our "comfortable" temp is 69 degrees. How much should this be lowered during day (at work) or night (asleep)? How much would be too much?

There is no real set standard for thermostat settings. Your best bet is to experiment around. Remember when it comes to the heating of your home with gas or oil the lower you set your thermostat the more energy savings. Also a steady state of lower temperature is always best since you have to pay $$$ to reheat the home to a higher temperature.

Years ago before the gas bills were so high I had a 70 year old widow that used to keep her thermostat set on 60 degrees all winter. When I asked her one day while servicing her old oil burner why she kept the temperature so low she answered that it was much cheaper to wear more clothes during the day than to buy more coal. Her father told he that during the depression when she was a young woman.

Many years ago a few of my customers still heated with coal. I remember removing clinkers and replacing sheer keys on many a cold night in the winter.

Many of us have no idea how nice we have it today with all the automatic devices especially the modern furnaces.