How recognize best sampling time with primary data set with 1 second sampling time?

H I have problem in signal processing, please help me every body that can help. I have a furnace, I would like recognize system model by System Identification approacehs, I don't know any system parameters such as Time constant, step response, cut off frequency and any other things, I just have Time domain data that is possible for me change sampling time from 1 second to any time that I like, I do not know which sampling time is better, becuase I must sampling from important system's dynamic for prepare good model, my frquency response of my system is swinging, Can you help me how can I recognize it? The attached picture show FFT plot of my tomain data with 1 second sampling time, I want to know can I have benefit dynamics of system with this sampling time or no? and if it is not good, how can recognize best sampling time from this FFT Plot? Best Regards.