The fastest I have been able to reliably switch between two BLE advertisements on iOS is 1 second. See my answer and sample code here.
The standard iOS iBeacon advertising frequency is 30 Hz, but in my experience, you only need to transmit an iBeacon advertisement every 900ms for fastest detection. More frequent advertisements are still useful for more accurate distance measurements, as iOS uses samples of RSSI from the iBeacon advertisements to make this estimate every second when ranging.