I'm dealing with a similar project now. Our ventilation was existing already so I didn't look for codes. If you still interested, here's what I learned:
The lower explosive limit for Hydrogen gas is 4% by volume. Using a safety factor of 5, (to account for gas production fluctuations, room temp, deterioration of ventilation system) this reduces to 0.8% by volume. Determine the H2 gas production rate. Determine the time in which 0.8% H2 gas concentration is reached. If, for example, it were 20 minutes, then at least one air change is needed per 20 minutes. To determine the H2 gas production rate I used the following:
(charging amperage) X (# of cells) X (.016 cubic feet of H2 per hour-cell-amp)