I tried to address this mathematically in a thread a couple of years ago: https://diysolarforum.com/threads/on-keeping-lfp-warm.17629/I did not find proper information regarding what energy is needed to heat up a battery in order to get it to a working temperature. So I did some experiments with my 48V 280Ah battery. With 1kWh I was able to heat it up from 2C to 22C. This is with proper heat insulation (6cm EPS) and in a 24-hour time window.
Let's consider the insulation perfect. The total cell weight is ~90kg. This means that you need ~1W of energy per kilogram of LiFePo4 cells to heat them by 1 degree Celsius.
Now let's consider the information on the internet about the LiFePo4 cells' specific heat energy. The numbers are at ~1000J/kg. Or roughly 0.3W per kilogram of cells to heat them up by 1 degree Celsius. Looks likely the insulation I have is far away from perfect.
This should give a rough estimate of the required heating time with a specific heating pad. I guess this would be useful when somebody is trying to decide what heating element is needed for his case.