I assume you are asking for how to model this, and not what the actual reaction time is- since it will be so dependent on thermal mass, geometry, and other factors. You should just be able to determine this from your energy model easily if you are using IES or Energy Plus- I'm not sure about eQuest or other softwares. They export hourly temperature data that you can plot. Just pulse on a load or increase the temperature setpoint for your period of interest and you can visualize the effect and determine the reaction time. I'm not sure if this is the best approach, as I've never done this study myself, but it is where I would start.