Who know the response time of hardware interrupt?

In my case, I link 4 didital interlock signals to the FM350-2 module in a S7-300 PLC.The four signals are nomarlly high,and maybe fall(1->0) in a interval of 30 micro seconds on by one.I want the PLCcan detect which signalfall (fall edge) first. If one signal fall, the FM350-2 will arouse a interruptto the S7-300, and then transfer OB40. I want to know the delay from one signal falling to transfering OB40.
Thank you!