[Outland]

"Any idea how long it takes for the H2 to leak out?"

I'm not sure. It all depends on the material used for the tank, it's thickness, any coatings applied, pressure, etc..

And in the words of guru Don Lancaster, there's more hydrogen in a gallon of gasoline than in a gallon of hydrogen.

[Prophet in the wilderness]

How about using a self sealing tanks like they use on jetliners ?
I don't know, using Hydrogen can't be any worse or dangerious than using propane or natural gas in a car.