If a tank instantaneously ruptures and completely releases hydrogen into the atmosphere, how long does it take to disperse the H2 to a point where it is no longer a fire or explosion risk?

This is an impossible question to answer without greater understanding of the quantities of hydrogen involved, the types of vessels involved, and the atmospheric conditions. Several companies offer software to model such releases. It’s important to note that there is a high probability of ignition either during the vessel rupture or from nearby ignition sources.