Hi
Wi-Fi Scanning is an interval is just how often it checks for networks when you aren't hooked to one.
I think that there is no absolute minimum scanning delay. It depends too much on the hardware performances.
For example if you limit it as 4 times in 2 minutes. So you could scan 4 times with 1 second of delay in between. But you would not get any further scan results for the next 126 seconds.
So the fastest interval where every scan is successful would be every 30 seconds.