The 25 us comes from the dt value of your waveforms, so I assume you are sampling your signals at 1/25 us = 40 kHz, right?
btw. I see that you have 3 filter options in your VI and that your default order is 1500, so for that one you should start your subset 1500 * 25 us = 37.5 ms after the beginning of your record.
As for arbitrary IIR filter, the task is not trivial in general, but if your response is relatively simple you can get away by playing with poles and zeros positions, but if you feel going on too deep water just stay with FIR. They seem to work for you.