IRVR values used for METAR and SPECI are 10 minute running averages. Live values displayed to ATC are 1 minute running averages.
But - the 1 minute average is only of the raw visibility, the MOR. Other RVR parameters, such as Background Luminance and Runway Light Intensity are monitored at least every 5 seconds and if they change the live IRVR will be updated immediately.
So if, for example, ATC change the brightness of the runway lights, the live IRVR value will change within a few seconds, the system will not wait until the next 1 minute MOR value is available.