I've been interested in this only because I'm datalogging our thermostat in mysql and have the numbers to mess around with. I'm trying to come up with a baseline number so I can compare before/after changes like attic insulation, new windows, etc that we plan on doing but in the end, there are just too many variables.
Sunlight hours, wind, whether we're home or out or in/out a lot, etc. We've had a few days lately where we've had over 6 hours runtime in a day, so that's 15 minutes out of each hour, but one hour solid has been getting the house back up to temperature in the morning from setback.
In the end, I just accept it for what it is, and make longterm decisions on how to improve things.