If a given amount of LH2 (liters or gallons) were to completely dump into air at standard temperature and pressure, what volume would gaseous H2 occupy?

It varies slightly due to different density of LH2 at different temperatures, but a gallon of LH2 at atmospheric pressure (0 psig) is ~113 SCF of H2. The expansion ratio is about 840:1. In metric units, a liter of LH2 at atmospheric pressure (0 MPa) would expand to about 840 liters of STP of gaseous pressure.