What does "maximum update interval" mean for Mode S GICB registers?
If you update the register, the values will be set to the values provided.
From that moment, a timer starts running. When the timer reaches 2 * maxUpdateInterval, the register will be set to all zero. 

If a radar extracts the register before the 2 * maxUpdateInterval is reached, it will get the values provided at the update. If it will extract the register after 2 * maxUpdateInterval is reached, it will get all zeros in the reply. 

For ADS-B, transmission of the register is inhibited (IIRC) when the register is set to all zeros.