I have all new windows, most walls are still R-7. I do have some R13 in some walls, a 3 season porch on the front corner, a sunroom off the back that is R19 in the walls, 1st floor is about 1500 sq ft now.
Our Temp falls about 4-8 degrees in 8 hours depending upon outside temps
I actually turn the heat down to 60 when I am the only one home
It was 69 degrees when my wife went out about 9am, its still 66 in here - sunny day but its only 25 outside. At times I will even shut the boiler off for the day, other wise it comes on 2-3 times. Probably not a good idea in the winter - since if I forget to turn it back on there could be a problem
At some point the house will lose heat at a much slower rate.
You would need an accurate heat loss calc & need to know how much it lost at different temp to figure this out. I just know having a programmable thermostat has saved us a lot on heating