That's exactly one of the questions I'm trying to answer!
I wouldn't think that you would need to collect data away from a PC for two days to determine that.
Get a volunteer subject to wear the device, while connected to a PC, and try different things to raise (strenuous exercise, exposure to heat, etc.) and lower (stick the subject in a walk in freezer) the subjects core temperature. Plot the temperature as you get it, and learn how quickly the core temperature changes under extreme conditions.
For people not subject to extreme conditions, the rate of change of core temperature is pretty low. Measuring the temperature every second will, I'm nearly positive, be unnecessary.
OI course, one of the hardest things to do with your test subject is going to be preventing outside factors from affecting the temperature reading that you get. That is, measuring what you think/hope you are measuring, and nothing else.